1 Samuel 24:6

What a virtuous action that was, when David wished rather to spare the king his enemy, though he could have injured him! How useful, too, it was, for it helped him when he succeeded to the throne. For all learned to be faithful to their king and not to seize the kingdom but to fear and reverence him. Thus what is virtuous was preferred to what was useful, and then usefulness followed on what was virtuous.

As a Christian judge, you must play the part of a loving father, you must show anger for wrongdoing but remember to make allowance for human weakness; do not indulge your inclination to seek vengeance for the vile acts of sinners, but direct your effort to the cure of the sinners' wounds.… There is also that well-known example of forbearance on the part of holy David, when his enemy was delivered into his hands and he spared him, an example which shines with greater luster from the fact that he had power to act otherwise. Do not, then, let your power of punishment make you harsh, when the necessity of inquiry did not shake your spirit of mildness.

It was not without God's influence, you see, that he [David] succeeded in prevailing over those frenzied men [his soldiers who wished to kill Saul]: the grace of God was found on the inspired man's lips, adding a sort of inducement to those words. It was, however, no slight contribution that David also made: since he had formed them in the past, consequently in the critical moment he found them ready and willing. It was not as leader of troops, you see, but as priest he commanded them, and that cave was a church on that occasion: like someone appointed as bishop, he delivered a homily to them, and after this homily he offered a kind of remarkable and unusual sacrifice, not sacrificing a calf, not slaying a lamb, but—what was of greater value than these—he offered to God gentleness and clemency, sacrificing irrational resentment, slaying anger and mortifying the limbs that are on the earth. He acted as victim, priest and altar: everything came from him—the thought that offered gentleness and clemency, the clemency and gentleness and the heart in which they were offered.