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-defense from fear of the future and by way of providing for his own security, no one would deprive him of the rewards of moderation. 

Nevertheless, David did not act even thus, but found a novel and strange form of moral wisdom. And neither the remembrance of things past, nor the fear of things to come, nor the instigation of the captain, nor the solitude of the place, nor the facility for slaying, nor anything else incited him to kill; but he spared the man who was his enemy and had given him pain, just as if he was some benefactor and had done him much good. What kind of indulgence then shall we have if we are mindful of past transgressions and avenge ourselves on those who have given us pain, whereas that innocent man who had undergone such great sufferings and expected more and worse evils to befall him in consequence of saving his enemy, is seen to spare him, so as to prefer incurring danger himself and to live in fear and trembling, rather than put to a just death the man who would cause him endless troubles?

His moral wisdom then we may perceive, not only from the fact that he did not slay Saul when there was so strong a compulsion, but also that he did not utter an irreverent word against him, although he who was insulted would not have heard him. Yet we often speak evil of friends when they are ab-sent, he on the contrary not even of the enemy who had done him such great wrong. His moral wisdom then we may perceive from these things, but his lovingkindness and tender care from what he did after these things. For when he had cut off the fringe of Saul’s garment and had taken away the bottle of water, he withdrew afar off and stood and shouted and exhibited these things to him whose life he had preserved, doing so not with a view to display and ostentation, but desiring to convince him by his deeds that he suspected him without a cause as his enemy, and aiming therefore at winning him into friendship. Nevertheless, when he had even thus failed to persuade him, and could have laid hands on him, he again chose rather to be an exile from his country and to sojourn in a strange land and suffer distress every day in procuring necessary food than to remain at home and vex his adversary. What spirit could be kinder than his? He was indeed justified in saying, “Lord remember David and all his meekness” (Ps. 131:1).

Saint John Chrysostom, If Thine Enemy Hunger, Feed Him
Homilies on Profitable Subjects