Theophylact of Ohrid

Do not feed your enemies with the thought that you will bring greater punishment upon them. The meaning of what is said is this: "be not overcome by evil," that is, do not do this with the intention of taking revenge, and do not imitate the evildoer (for that means being overcome), but strive rather to overcome him by your own benevolence and to turn him away from malice. Thus, what was said above he spoke for the encouragement of the fainthearted, but now he has set forth what is more perfect.

Commentary on Romans ↗

Cyprian

That evil is not to be returned for evil. In the Epistle of Paul to the Romans: "Rendering to no man evil for evil." Also in the same place: "Not to be overcome of evil, but overcome evil with good." Of this same thing in the Apocalypse: "And He said unto me, Seal not the words of the prophecy of this book; because now the time is at hand. …

Treatise XII Three Books of Testimonies Against the Jews ↗

Desert Fathers

A brother asked a hermit, 'What is humility?' He answered, 'To do good to them that do evil to you.' The brother said, 'Suppose a man cannot attain that standard, what is he to do?' The hermit answered, 'He should run away, and choose silence.'

The Desert Fathers, Sayings of the Early Christian Monks ↗

Augustine of Hippo

The evil man who is overcome by good is set free, not from an exterior, foreign evil but from an interior, personal one, by which he is more grievously and ruinously laid waste than he would be by the inhumanity of any enemy from without.

Letter 138 ↗

Thomas Aquinas

Then when he says, do not be overcome by evil, he proves what he had said by reason. For it is natural to man that he wish to overcome his adversary and not be overcome by him. But that is overcome by another which is drawn to that other, as water is overcome by fire, when it is drawn to the heat of fire. Therefore, if a good man, on account of the evil done him by another, is drawn to do evil in return, the good man is overcome by the evil. …

Commentary on Romans ↗