Augustine of Hippo — as excerpted in the Catena Aurea on Matthew 5:38-42

“(cont. Faust. xix. 25.) This law, Eye for eye, tooth for tooth, was enacted to repress the flames of mutual hate, and to be a check on their undisciplined spirits. For who when he would take revenge, was ever content to return just so much harm as he had received? Do we not see men who have suffered some trifling hurt, straightway plot murder, thirst for blood, and hardly find evil enough that they can do to their enemies for the satisfying their rage? To this immeasured and cruel fury the Law puts bounds when it enacts a lex talionis; that is, that whatever wrong or hurt any man has done to another, he should suffer just the same in return. This is not to encourage but to check rage; for it does not rekindle what was extinguished, but hinders the flames already kindled from further spread. It enacts a just retaliation, properly due to him who has suffered the wrong. But that mercy forgives any debt, does not make it unjust that payment had been sought. Since then he sins who seeks an unmeasured vengeance, but he does not sin who desires only a just one; he is therefore further from sin who seeks no retribution at all. I might state it yet thus; It was said to them of old time, Thou shalt not take unequal retaliation; But I say unto you. Ye shall not retaliate; this is a completion of the Law, if in these words something is added to the Law which was wanting to it; yea, rather that which the Law sought to do, namely, to put an end to unequal revenge, is more safely secured when there is no revenge at all.”