Summa Th. II-II EN Qu.106 a.3

Whether a man is bound to give thanks to every benefactor?

Objection: 1. It seems that the a man is not bound to give thanks to every benefactor. For a man may benefit himself just as he may harm himself, according to Si 14,5, "He that is evil to himself, to whom will he be good?" But a man cannot thank himself, since thanksgiving seems to pass from one person to another. Therefore thanksgiving is not due to every benefactor.
2. Further, gratitude is a repayment of an act of grace. But some favors are granted without grace, and are rudely, slowly and grudgingly given. Therefore gratitude is not always due to a benefactor.
3. Further, no thanks are due to one who works for his own profit. But sometimes people bestow favors for their own profit. Therefore thanks are not due to them.
4. Further, no thanks are due to a slave, for all that he is belongs to his master. Yet sometimes a slave does a good turn to his master. Therefore gratitude is not due to every benefactor .
5. Further, no one is bound to do what he cannot do equitably and advantageously. Now it happens at times that the benefactor is very well off, and it would be of no advantage to him to be repaid for a favor he has bestowed. Again it happens sometimes that the benefactor from being virtuous has become wicked, so that it would not seem equitable to repay him. Also the recipient of a favor may be a poor man, and is quite unable to repay. Therefore seemingly a man is not always bound to repayment for favors received.
6. Further, no one is bound to do for another what is inexpedient and hurtful to him. Now sometimes it happens that repayment of a favor would be hurtful or useless to the person repaid. Therefore favors are not always to be repaid by gratitude.

On the contrary It is written (1Th 5,18): "In all things give thanks."
I answer that Every effect turns naturally to its cause; wherefore Dionysius says (Div. Nom. i) that "God turns all things to Himself because He is the cause of all": for the effect must needs always be directed to the end of the agent. Now it is evident that a benefactor, as such, is cause of the beneficiary. Hence the natural order requires that he who has received a favor should, by repaying the favor, turn to his benefactor according to the mode of each. And, as stated above with regard to a father (Question [31], Article [3]; Question [101], Article [2]), a man owes his benefactor, as such, honor and reverence, since the latter stands to him in the relation of principle; but accidentally he owes him assistance or support, if he need it.

Reply to Objection: 1. In the words of Seneca (1 Benef. v), "just as a man is liberal who gives not to himself but to others, and gracious who forgives not himself but others, and merciful who is moved, not by his own misfortunes but by another's, so too, no man confers a favor on himself, he is but following the bent of his nature, which moves him to resist what hurts him, and to seek what is profitable." Wherefore in things that one does for oneself, there is no place for gratitude or ingratitude, since a man cannot deny himself a thing except by keeping it. Nevertheless things which are properly spoken of in relation to others are spoken of metaphorically in relation to oneself, as the Philosopher states regarding justice (Ethic. v, 11), in so far, to wit, as the various parts of man are considered as though they were various persons.
2. It is the mark of a happy disposition to see good rather than evil. Wherefore if someone has conferred a favor, not as he ought to have conferred it, the recipient should not for that reason withhold his thanks. Yet he owes less thanks, than if the favor had been conferred duly, since in fact the favor is less, for, as Seneca remarks (De Benef. ii.) "promptness enhances, delay discounts a favor."
3. As Seneca observes (De Benef. vi), "it matters much whether a person does a kindness to us for his own sake, or for ours, or for both his and ours. He that considers himself only, and benefits because cannot otherwise benefit himself, seems to me like a man who seeks fodder for his cattle." And farther on: "If he has done it for me in common with himself, having both of us in his mind, I am ungrateful and not merely unjust, unless I rejoice that what was profitable to him is profitable to me also. It is the height of malevolence to refuse to recognize a kindness, unless the giver has been the loser thereby."
4. As Seneca observes (De Benef. iii), "when a slave does what is wont to be demanded of a slave, it is part of his service: when he does more than a slave is bound to do, it is a favor: for as soon as he does anything from a motive of friendship, if indeed that be his motive, it is no longer called service." Wherefore gratitude is due even to a slave, when he does more than his duty.
5. A poor man is certainly not ungrateful if he does what he can. For since kindness depends on the heart rather than on the deed, so too gratitude depends chiefly the heart. Hence Seneca says (De Benef. ii): "Who receives a favor gratefully, has already begun to pay it back: and that we are grateful for favors received should be shown by the outpourings of the heart, not only in his hearing but everywhere." From this it is evident that however well off a man may be, it is possible to thank him for his kindness by showing him reverence and honor. Wherefore the Philosopher says (Ethic. viii, 14): "He that abounds should be repaid with honor, he that is in want should be repaid with money": and Seneca writes (De Benef. vi): "There are many ways of repaying those who are well off, whatever we happen to owe them; such as good advice, frequent fellowship, affable and pleasant conversation without flattery." Therefore there is no need for a man to desire neediness or distress in his benefactor before repaying his kindness, because, as Seneca says (De Benef. vi), "it were inhuman to desire this in one from whom you have received no favor; how much more so to desire it in one whose kindness has made you his debtor!"If, however, the benefactor has lapsed from virtue, nevertheless he should be repaid according to his state, that he may return to virtue if possible. But if he be so wicked as to be incurable, then his heart has changed, and consequently no repayment is due for his kindness, as heretofore. And yet, as far as it possible without sin, the kindness he has shown should be held in memory, as the Philosopher says (Ethic. ix, 3).
6. As stated in the preceding reply, repayment of a favor depends chiefly on the affection of the heart: wherefore repayment should be made in such a way as to prove most beneficial. If, however, through the benefactor's carelessness it prove detrimental to him, this is not imputed to the person who repays him, as Seneca observes (De Benef. vii): "It is my duty to repay, and not to keep back and safeguard my repayment."

Whether a man is bound to repay a favor at once?

Objection: 1. It seems that a man is bound to repay a favor at once. For we are bound to restore at once what we owe, unless the term be fixed. Now there is no term prescribed for the repayment of favors, and yet this repayment is a duty, as stated above (Article [3]). Therefore a man is bound to repay a favor at once.
2. Further, a good action would seem to be all the more praiseworthy according as it is done with greater earnestness. Now earnestness seems to make a man do his duty without any delay. Therefore it is apparently more praiseworthy to repay a favor at once.
3. Further, Seneca says (De Benef. ii) that "it is proper to a benefactor to act freely and quickly." Now repayment ought to equal the favor received. Therefore it should be done at once.

On the contrary Seneca says (De Benef. iv): "He that hastens to repay, is animated with a sense, not of gratitude but of indebtedness."
I answer that Just as in conferring a favor two things are to be considered, namely, the affection of the heart and the gift, so also must these things be considered in repaying the favor. As regards the affection of the heart, repayment should be made at once, wherefore Seneca says (De Benef. ii): "Do you wish to repay a favor? Receive it graciously." As regards the gift, one ought to wait until such a time as will be convenient to the benefactor. In fact, if instead of choosing a convenient time, one wished to repay at once, favor for favor, it would not seem to be a virtuous, but a constrained repayment. For, as Seneca observes (De Benef. iv), "he that wishes to repay too soon, is an unwilling debtor, and an unwilling debtor is ungrateful."

Reply to Objection: 1. A legal debt must be paid at once, else the equality of justice would not be preserved, if one kept another's property without his consent. But a moral debt depends on the equity of the debtor: and therefore it should be repaid in due time according as the rectitude of virtue demands.
2. Earnestness of the will is not virtuous unless it be regulated by reason; wherefore it is not praiseworthy to forestall the proper time through earnestness.
3. Favors also should be conferred at a convenient time and one should no longer delay when the convenient time comes; and the same is to be observed in repaying favors.

Whether in giving thanks we should look at the benefactor's disposition or at the deed?

Objection: 1. It seems that in repaying favors we should not look at the benefactor's disposition but at the deed. For repayment is due to beneficence, and beneficence consists in deeds, as the word itself denotes. Therefore in repaying favors we should look at the deed.
2. Further, thanksgiving, whereby we repay favors, is a part of justice. But justice considers equality between giving and taking. Therefore also in repaying favors we should consider the deed rather than the disposition of the benefactor.
3. Further, no one can consider what he does not know. Now God alone knows the interior disposition. Therefore it is impossible to repay a favor according to the benefactor's disposition.

On the contrary Seneca says (De Benef. i): "We are sometimes under a greater obligation to one who has given little with a large heart, and has bestowed a small favor, yet willingly."
I answer that The repayment of a favor may belong to three virtues, namely, justice, gratitude and friendship. It belongs to justice when the repayment has the character of a legal debt, as in a loan and the like: and in such cases repayment must be made according to the quantity received.On the other hand, repayment of a favor belongs, though in different ways, to friendship and likewise to the virtue of gratitude when it has the character of a moral debt. For in the repayment of friendship we have to consider the cause of friendship; so that in the friendship that is based on the useful, repayment should be made according to the usefulness accruing from the favor conferred, and in the friendship based on virtue repayment should be made with regard for the choice or disposition of the giver, since this is the chief requisite of virtue, as stated in Ethic. viii, 13. And likewise, since gratitude regards the favor inasmuch as it is bestowed gratis, and this regards the disposition of the giver, it follows again that repayment of a favor depends more on the disposition of the giver than on the effect.

Reply to Objection: 1. Every moral act depends on the will. Hence a kindly action, in so far as it is praiseworthy and is deserving of gratitude, consists materially in the thing done, but formally and chiefly in the will. Hence Seneca says (De Benef. i): "A kindly action consists not in deed or gift, but in the disposition of the giver or doer."
2. Gratitude is a part of justice, not indeed as a species is part of a genus, but by a kind of reduction to the genus of justice, as stated above (Question [80]). Hence it does not follow that we shall find the same kind of debt in both virtues.
3. God alone sees man's disposition in itself: but in so far as it is shown by certain signs, man also can know it. It is thus that a benefactor's disposition is known by the way in which he does the kindly action, for instance through his doing it joyfully and readily.

Whether the repayment of gratitude should surpass the favor received?

Objection: 1. It seems that there is no need for the repayment of gratitude to surpass the favor received. For it is not possible to make even equal repayment to some, for instance, one's parents, as the Philosopher states (Ethic. viii, 14). Now virtue does not attempt the impossible. Therefore gratitude for a favor does not tend to something yet greater.
2. Further, if one person repays another more than he has received by his favor, by that very fact he gives him something his turn, as it were. But the latter owes him repayment for the favor which in his turn the former has conferred on him. Therefore he that first conferred a favor will be bound to a yet greater repayment, and so on indefinitely. Now virtue does not strive at the indefinite, since "the indefinite removes the nature of good" (Metaph. ii, text. 8). Therefore repayment of gratitude should not surpass the favor received.
3. Further, justice consists in equality. But "more" is excess of equality. Since therefore excess is sinful in every virtue, it seems that to repay more than the favor received is sinful and opposed to justice.

On the contrary The Philosopher says (Ethic. v, 5): "We should repay those who are gracious to us, by being gracious to them return," and this is done by repaying more than we have received. Therefore gratitude should incline to do something greater.
I answer that As stated above (Article [5]), gratitude regards the favor received according the intention of the benefactor; who seems be deserving of praise, chiefly for having conferred the favor gratis without being bound to do so. Wherefore the beneficiary is under a moral obligation to bestow something gratis in return. Now he does not seem to bestow something gratis, unless he exceeds the quantity of the favor received: because so long as he repays less or an equivalent, he would seem to do nothing gratis, but only to return what he has received. Therefore gratitude always inclines, as far as possible, to pay back something more.

Reply to Objection: 1. As stated above (Article [3], ad 5; Article [5]), in repaying favors we must consider the disposition rather than the deed. Accordingly, if we consider the effect of beneficence, which a son receives from his parents namely, to be and to live, the son cannot make an equal repayment, as the Philosopher states (Ethic. viii, 14). But if we consider the will of the giver and of the repayer, then it is possible for the son to pay back something greater to his father, as Seneca declares (De Benef. iii). If, however, he were unable to do so, the will to pay back would be sufficient for gratitude.
2. The debt of gratitude flows from charity, which the more it is paid the more it is due, according to Rm 13,8, "Owe no man anything, but to love one another." Wherefore it is not unreasonable if the obligation of gratitude has no limit.
3. As in injustice, which is a cardinal virtue, we consider equality of things, so in gratitude we consider equality of wills. For while on the one hand the benefactor of his own free-will gave something he was not bound to give, so on the other hand the beneficiary repays something over and above what he has received.


We must now consider ingratitude, under which head there are four points of inquiry:

(1) Whether ingratitude is always a sin?

(2) Whether ingratitude is a special sin?

(3) Whether every act of ingratitude is a mortal sin?

(4) Whether favors should be withdrawn from the ungrateful?

Whether ingratitude is always a sin?

Objection: 1. It seems that ingratitude is not always a sin. For Seneca says (De Benef. iii) that "he who does not repay a favor is ungrateful." But sometimes it is impossible to repay a favor without sinning, for instance if one man has helped another to commit a sin. Therefore, since it is not a sin to refrain from sinning, it seems that ingratitude is not always a sin.
2. Further, every sin is in the power of the person who commits it: because, according to Augustine (De Lib. Arb. iii; Retract. i), "no man sins in what he cannot avoid." Now sometimes it is not in the power of the sinner to avoid ingratitude, for instance when he has not the means of repaying. Again forgetfulness is not in our power, and yet Seneca declares (De Benef. iii) that "to forget a kindness is the height of ingratitude." Therefore ingratitude is not always a sin.
3. Further, there would seem to be no repayment in being unwilling to owe anything, according to the Apostle (Rm 13,8), "Owe no man anything." Yet "an unwilling debtor is ungrateful," as Seneca declares (De Benef. iv). Therefore ingratitude is not always a sin.

On the contrary Ingratitude is reckoned among other sins (2Tm 3,2), where it is written: "Disobedient to parents, ungrateful, wicked." etc.
I answer that As stated above (Question [106], Article [4], ad 1, Article [6]) a debt of gratitude is a moral debt required by virtue. Now a thing is a sin from the fact of its being contrary to virtue. Wherefore it is evident that every ingratitude is a sin.

Reply to Objection: 1. Gratitude regards a favor received: and he that helps another to commit a sin does him not a favor but an injury: and so no thanks are due to him, except perhaps on account of his good will, supposing him to have been deceived, and to have thought to help him in doing good, whereas he helped him to sin. In such a case the repayment due to him is not that he should be helped to commit a sin, because this would be repaying not good but evil, and this is contrary to gratitude.
2. No man is excused from ingratitude through inability to repay, for the very reason that the mere will suffices for the repayment of the debt of gratitude, as stated above (Question [106], Article [6], ad 1).Forgetfulness of a favor received amounts to ingratitude, not indeed the forgetfulness that arises from a natural defect, that is not subject to the will, but that which arises from negligence. For, as Seneca observes (De Benef. iii), "when forgetfulness of favors lays hold of a man, he has apparently given little thought to their repayment."
3. The debt of gratitude flows from the debt of love, and from the latter no man should wish to be free. Hence that anyone should owe this debt unwillingly seems to arise from lack of love for his benefactor.

Whether ingratitude is a special sin?

Objection: 1. It seems that ingratitude is not a special sin. For whoever sins acts against God his sovereign benefactor. But this pertains to ingratitude. Therefore ingratitude is not a special sin.
2. Further, no special sin is contained under different kinds of sin. But one can be ungrateful by committing different kinds of sin, for instance by calumny, theft, or something similar committed against a benefactor. Therefore ingratitude is not a special sin.
3. Further, Seneca writes (De Benef. iii): "It is ungrateful to take no notice of a kindness, it is ungrateful not to repay one, but it is the height of ingratitude to forget it." Now these do not seem to belong to the same species of sin. Therefore ingratitude is not a special sin.

On the contrary Ingratitude is opposed to gratitude or thankfulness, which is a special virtue. Therefore it is a special sin.
I answer that Every vice is denominated from a deficiency of virtue, because deficiency is more opposed to virtue: thus illiberality is more opposed to liberality than prodigality is. Now a vice may be opposed to the virtue of gratitude by way of excess, for instance if one were to show gratitude for things for which gratitude is not due, or sooner than it is due, as stated above (Question [106], Article [4]). But still more opposed to gratitude is the vice denoting deficiency of gratitude, because the virtue of gratitude, as stated above (Question [106], Article [6]), inclines to return something more. Wherefore ingratitude is properly denominated from being a deficiency of gratitude. Now every deficiency or privation takes its species from the opposite habit: for blindness and deafness differ according to the difference of sight and hearing. Therefore just as gratitude or thankfulness is one special virtue, so also is ingratitude one special sin.It has, however, various degrees corresponding in their order to the things required for gratitude. The first of these is to recognize the favor received, the second to express one's appreciation and thanks, and the third to repay the favor at a suitable place and time according to one's means. And since what is last in the order of generation is first in the order of destruction, it follows that the first degree of ingratitude is when a man fails to repay a favor, the second when he declines to notice or indicate that he has received a favor, while the third and supreme degree is when a man fails to recognize the reception of a favor, whether by forgetting it or in any other way. Moreover, since opposite affirmation includes negation, it follows that it belongs to the first degree of ingratitude to return evil for good, to the second to find fault with a favor received, and to the third to esteem kindness as though it were unkindness.

Reply to Objection: 1. In every sin there is material ingratitude to God, inasmuch as a man does something that may pertain to ingratitude. But formal ingratitude is when a favor is actually contemned, and this is a special sin.
2. Nothing hinders the formal aspect of some special sin from being found materially in several kinds of sin, and in this way the aspect of ingratitude is to be found in many kinds of sin.
3. These three are not different species but different degrees of one special sin.

Whether ingratitude is always a mortal sin?

Objection: 1. It seems that ingratitude is always a mortal sin. For one ought to be grateful to God above all. But one is not ungrateful to God by committing a venial sin: else every man would be guilty of ingratitude. Therefore no ingratitude is a venial sin.
2. Further, a sin is mortal through being contrary to charity, as stated above (Question [24], Article [12]). But ingratitude is contrary to charity, since the debt of gratitude proceeds from that virtue, as stated above (Question [106], Article [1], ad 3; Article [6], ad 2). Therefore ingratitude is always a mortal sin.
3. Further, Seneca says (De Benef. ii): "Between the giver and the receiver of a favor there is this law, that the former should forthwith forget having given, and the latter should never forget having received." Now, seemingly, the reason why the giver should forget is that he may be unaware of the sin of the recipient, should the latter prove ungrateful; and there would be no necessity for that if ingratitude were a slight sin. Therefore ingratitude is always a mortal sin.

On the contrary On the contrary, No one should be put in the way of committing a mortal sin. Yet, according to Seneca (De Benef. ii), "sometimes it is necessary to deceive the person who receives assistance, in order that he may receive without knowing from whom he has received." But this would seem to put the recipient in the way of ingratitude. Therefore ingratitude is not always a mortal sin.
I answer that As appears from what we have said above (Article [2]), a man may be ungrateful in two ways: First, by mere omission, for instance by failing to recognize the favor received, or to express his appreciation of it or to pay something in return, and this is not always a mortal sin, because, as stated above (Question [106], Article [6]), the debt of gratitude requires a man to make a liberal return, which, however, he is not bound to do; wherefore if he fail to do so, he does not sin mortally. It is nevertheless a venial sin, because it arises either from some kind of negligence or from some disinclination to virtue in him. And yet ingratitude of this kind may happen to be a mortal sin, by reason either of inward contempt, or of the kind of thing withheld, this being needful to the benefactor, either simply, or in some case of necessity.Secondly, a man may be ungrateful, because he not only omits to pay the debt of gratitude, but does the contrary. This again is sometimes mortal and sometimes a venial sin, according to the kind of thing that is done.It must be observed, however, that when ingratitude arises from a mortal sin, it has the perfect character of ingratitude, and when it arises from venial sin, it has the imperfect character.

Reply to Objection: 1. By committing a venial sin one is not ungrateful to God to the extent of incurring the guilt of perfect ingratitude: but there is something of ingratitude in a venial sin, in so far as it removes a virtuous act of obedience to God.
2. When ingratitude is a venial sin it is not contrary to, but beside charity: since it does not destroy the habit of charity, but excludes some act thereof.
3. Seneca also says (De Benef. vii): "When we say that a man after conferring a favor should forget about it, it is a mistake to suppose that we mean him to shake off the recollection of a thing so very praiseworthy. When we say: He must not remember it, we mean that he must not publish it abroad and boast about it."
4. He that is unaware of a favor conferred on him is not ungrateful, if he fails to repay it, provided he be prepared to do so if he knew. It is nevertheless commendable at times that the object of a favor should remain in ignorance of it, both in order to avoid vainglory, as when Blessed Nicolas threw gold into a house secretly, wishing to avoid popularity: and because the kindness is all the greater through the benefactor wishing not to shame the person on whom he is conferring the favor.

Whether favors should be withheld from the ungrateful?

Objection: 1. It seems that favors should withheld from the ungrateful. For it is written (Sg 16,29): "The hope of the unthankful shall melt away as the winter's ice." But this hope would not melt away unless favors were withheld from him. Therefore favors should be withheld from the ungrateful.
2. Further, no one should afford another an occasion of committing sin. But the ungrateful in receiving a favor is given an occasion of ingratitude. Therefore favors should not be bestowed on the ungrateful.
3. Further, "By what things a man sinneth, by the same also he is tormented" (Sg 11,17). Now he that is ungrateful when he receives a favor sins against the favor. Therefore he should be deprived of the favor.

On the contrary It is written (Lc 6,35) that "the Highest . . . is kind to the unthankful, and to the evil." Now we should prove ourselves His children by imitating Him (Lc 6,36). Therefore we should not withhold favors from the ungrateful.
I answer that There are two points to be considered with regard to an ungrateful person. The first is what he deserves to suffer and thus it is certain that he deserves to be deprived of our favor. The second is, what ought his benefactor to do? For in the first place he should not easily judge him to be ungrateful, since, as Seneca remarks (De Benef. iii), "a man is often grateful although he repays not," because perhaps he has not the means or the opportunity of repaying. Secondly, he should be inclined to turn his ungratefulness into gratitude, and if he does not achieve this by being kind to him once, he may by being so a second time. If, however, the more he repeats his favors, the more ungrateful and evil the other becomes, he should cease from bestowing his favors upon him.

Reply to Objection: 1. The passage quoted speaks of what the ungrateful man deserves to suffer.
2. He that bestows a favor on an ungrateful person affords him an occasion not of sin but of gratitude and love. And if the recipient takes therefrom an occasion of ingratitude, this is not to be imputed to the bestower.
3. He that bestows a favor must not at once act the part of a punisher of ingratitude, but rather that of a kindly physician, by healing the ingratitude with repeated favors.


We must now consider vengeance, under which head there are four points of inquiry:

(1) Whether vengeance is lawful?

(2) Whether it is a special virtue?

(3) Of the manner of taking vengeance;

(4) On whom should vengeance be taken?

Whether vengeance is lawful?

Objection: 1. It seems that vengeance is not lawful. For whoever usurps what is God's sins. But vengeance belongs to God, for it is written (Dt 32,35, Rm 12,19): "Revenge to Me, and I will repay." Therefore all vengeance is unlawful.
2. Further, he that takes vengeance on a man does not bear with him. But we ought to bear with the wicked, for a gloss on Ct 2,2, "As the lily among the thorns," says: "He is not a good man that cannot bear with a wicked one." Therefore we should not take vengeance on the wicked.
3. Further, vengeance is taken by inflicting punishment, which is the cause of servile fear. But the New Law is not a law of fear, but of love, as Augustine states (Contra Adamant. xvii). Therefore at least in the New Testament all vengeance is unlawful.
4. Further, a man is said to avenge himself when he takes revenge for wrongs inflicted on himself. But, seemingly, it is unlawful even for a judge to punish those who have wronged him: for Chrysostom [*Cf. Opus Imperfectum, Hom. v in Matth., falsely ascribed to St. Chrysostom] says: "Let us learn after Christ's example to bear our own wrongs with magnanimity, yet not to suffer God's wrongs, not even by listening to them." Therefore vengeance seems to be unlawful.
5. Further, the sin of a multitude is more harmful than the sin of only one: for it is written (Si 26,5-7): "Of three things my heart hath been afraid . . . the accusation of a city, and the gathering together of the people, and a false calumny." But vengeance should not be taken on the sin of a multitude, for a gloss on Mt 13,29-30, "Lest perhaps . . . you root up the wheat . . . suffer both to grow," says that "a multitude should not be excommunicated, nor should the sovereign." Neither therefore is any other vengeance lawful.

On the contrary We should look to God for nothing save what is good and lawful. But we are to look to God for vengeance on His enemies: for it is written (Lc 18,7): "Will not God revenge His elect who cry to Him day and night?" as if to say: "He will indeed." Therefore vengeance is not essentially evil and unlawful.
I answer that Vengeance consists in the infliction of a penal evil on one who has sinned. Accordingly, in the matter of vengeance, we must consider the mind of the avenger. For if his intention is directed chiefly to the evil of the person on whom he takes vengeance and rests there, then his vengeance is altogether unlawful: because to take pleasure in another's evil belongs to hatred, which is contrary to the charity whereby we are bound to love all men. Nor is it an excuse that he intends the evil of one who has unjustly inflicted evil on him, as neither is a man excused for hating one that hates him: for a man may not sin against another just because the latter has already sinned against him, since this is to be overcome by evil, which was forbidden by the Apostle, who says (Rm 12,21): "Be not overcome by evil, but overcome evil by good."If, however, the avenger's intention be directed chiefly to some good, to be obtained by means of the punishment of the person who has sinned (for instance that the sinner may amend, or at least that he may be restrained and others be not disturbed, that justice may be upheld, and God honored), then vengeance may be lawful, provided other due circumstances be observed.

Reply to Objection: 1. He who takes vengeance on the wicked in keeping with his rank and position does not usurp what belongs to God but makes use of the power granted him by God. For it is written (Rm 13,4) of the earthly prince that "he is God's minister, an avenger to execute wrath upon him that doeth evil." If, however, a man takes vengeance outside the order of divine appointment, he usurps what is God's and therefore sins.
2. The good bear with the wicked by enduring patiently, and in due manner, the wrongs they themselves receive from them: but they do not bear with them as to endure the wrongs they inflict on God and their neighbor. For Chrysostom [*Cf. Opus Imperfectum, Hom. v in Matth., falsely ascribed to St. Chrysostom] says: "It is praiseworthy to be patient under our own wrongs, but to overlook God's wrongs is most wicked."
3. The law of the Gospel is the law of love, and therefore those who do good out of love, and who alone properly belong to the Gospel, ought not to be terrorized by means of punishment, but only those who are not moved by love to do good, and who, though they belong to the Church outwardly, do not belong to it in merit.
4. Sometimes a wrong done to a person reflects on God and the Church: and then it is the duty of that person to avenge the wrong. For example, Elias made fire descend on those who were come to seize him (2R 1,9); likewise Eliseus cursed the boys that mocked him (2R 2,23); and Pope Sylverius excommunicated those who sent him into exile (XXIII, Q. iv, Cap. Guilisarius). But in so far as the wrong inflicted on a man affects his person, he should bear it patiently if this be expedient. For these precepts of patience are to be understood as referring to preparedness of the mind, as Augustine states (De Serm. Dom. in Monte i).
5. When the whole multitude sins, vengeance must be taken on them, either in respect of the whole multitude---thus the Egyptians were drowned in the Red Sea while they were pursuing the children of Israel (Ex 14,22), and the people of Sodom were entirely destroyed (Gn 19)---or as regards part of the multitude, as may be seen in the punishment of those who worshipped the calf.Sometimes, however, if there is hope of many making amends, the severity of vengeance should be brought to bear on a few of the principals, whose punishment fills the rest with fear; thus the Lord (Num 25) commanded the princes of the people to be hanged for the sin of the multitude.On the other hand, if it is not the whole but only a part of the multitude that has sinned, then if the guilty can be separated from the innocent, vengeance should be wrought on them: provided, however, that this can be done without scandal to others; else the multitude should be spared and severity foregone. The same applies to the sovereign, whom the multitude follow. For his sin should be borne with, if it cannot be punished without scandal to the multitude: unless indeed his sin were such, that it would do more harm to the multitude, either spiritually or temporally, than would the scandal that was feared to arise from his punishment.

Summa Th. II-II EN Qu.106 a.3