This law, 'Eye for eye, tooth for tooth' (lex talionis), 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 of 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. 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.
Note that Jesus does not say, 'Give all things to him that asks', but, 'Give to the one who asks of you'; that you should only give what you can give honestly and rightly. For what if one ask for money to employ in oppressing the innocent man? What if he ask your consent to unclean in? We must give then only what will hurt neither ourselves or others, as far as man can judge; and when you have refused an inadmissible request, that you may not send away empty him that asked, show the righteousness of your refusal; and such correction of the unlawful petitioner will often be a better gift than the granting of his suit.
Some object that this command of Christ is altogether inconsistent with civil life in Commonwealths; who, say they, would suffer, when he could hinder it, the pillage of his estate by an enemy; or would not repay the evil suffered by a plundered province of Rome on the plunderers according to the rights of war? But these precepts of patience are to be observed in readiness of the heart, and that mercy, not to return evil for evil, must be always fulfilled by the will. Yet must we often use a merciful sharpness in dealing with the headstrong. And in this way, if the earthly commonwealth will keep the Christian commandments, even war will not be waged without good charities, to the establishing among the vanquished peaceful harmony of godliness and righteousness. For that victory is beneficial to him from whom it snatches license to sin; since nothing is more unfortunate for sinners, than the good fortune of their sins, which nourishes an impunity that brings punishment after it, and an evil will is strengthened, as it were some internal enemy.
Contra Faustum, Bk. XIX.25 (St. Augustine)
On the Sermon of the Mount, Bk. 1, Ch. 19 (St. Augustine)
Letter 138, Ch. 2 (St. Augustine)