Well! Did the captain who then counselled and urged him to the deed, did the memory of past events induce him to slay? No one of these things moved him: in fact the very facility for slaughter averted him from it: for he bethought him that God had put Saul in his hands for the purpose of furnishing ample ground and opportunity for the exercise of moral wisdom. You then perhaps admire him, because he did not cherish the memory of any of his past evils: but I am much more astonished at him for another reason.
And what is this? That the fear of future events did not impel him to lay violent hands on his enemy. For he knew clearly that if Saul escaped his hands, he would again be his adversary; yet he preferred exposing himself to danger by letting go the man who had wronged him, to providing for his own security by laying violent hands upon his foe. What could equal then the great and generous spirit of this man, who, when the law commanded eye to be plucked out for eye, and tooth for tooth, and retaliation on equal terms, not only abstained from doing this, but exhibited a far greater measure of moral wisdom?
At least if he had slain Saul at that time he would have retained credit for moral wisdom unimpaired, not merely because he had acted on the defensive, not being himself the originator of violence, but also because by his great moderation he was superior to the precept “an eye for an eye.” For he would not have inflicted one slaughter in return for one; but, in return for many deaths, which Saul endeavoured to bring on him, having attempted to slay him not once or twice but many times, he would have brought only one death on Saul; and not only this, but if he had proceeded to avenge himself out of fear of the future, even this, combined with the things already mentioned, would procure him the reward of forbearance without any deduction.
For he who is angry on account of the things which have been done to him, and demands satisfaction would not be able to obtain the praise of forbearance: but when a man dismisses the consideration of all past evils, although they are many and painful, but is compelled to take steps for self-defence from fear of the future, and by way of providing for his own security, no one would deprive him of the rewards of moderation.
Source: Homily on the Passage "If your enemy hunger, feed him." (New Advent)