1 Samuel 24:5

This is why the coming together of Saul, who was in pursuit of murder, and of David, who was shunning murder, in the cave is described after many events which it had preceded. The authority to kill was reversed in this event, since the one who was being pursued for execution had authority over the slaughter of his killer, and although he had the right, so far as retribution against his enemy was concerned, he stayed his power so far as consisted with the right and killed his own anger in himself instead of his enemy. - "On the Inscriptions of the Psalms 2.14.224"

When, therefore, he was alone and had removed his robe and laid it down somewhere beside him, the light from the mouth of the cave made him visible to those hidden within the darkness. All of David's companions wanted to rush upon the enemy and avenge themselves on the one who had come to kill them, since God had given the enemy into the hands of those who were being pursued for slaughter. But David forbade them to attack, considering an assault against their king unlawful. He drew his own sword from its sheath and imperceptibly stood behind Saul. There was no witness to his undertaking against Saul, for the darkness in the cave concealed his appearance and prevented scrutiny of what was happening. When, then, he could have driven his whole sword through Saul's heart from behind with one blow, he neither touched his body nor was he about to. But he secretly cut off the end of his robe with his sword, so that the garment might be a witness later of his clemency toward Saul and prove the power that he had had to strike a blow against Saul's body by means of the cut at its end.By this it became obvious that David had been trained in forbearance. When he held the bare sword in his palm, and the body of his enemy lay under his hand, he had the power to kill him, but he conquered his anger with reason, and his power to strike the blow with the fear of God. Not only did he become superior to his own anger, but he also restrained his shield bearer who was eager to murder Saul, addressing him with that saying famous in song, "Destroy not the anointed of the Lord."

In this way, too, the kingdom of Saul himself, who certainly was reprobated and rejected, was the shadow of a kingdom yet to come which should remain to eternity. For, indeed, the oil with which he was anointed, and from that chrism he is called Christ, is to be taken in a mystical sense, and is to be understood as a great mystery; which David himself venerated so much in him, that he trembled with smitten heart when, being hid in a dark cave, which Saul also entered when pressed by the necessity of nature, he had come secretly behind him and cut off a small piece of his robe, that he might be able to prove how he had spared him when he could have killed him, and might thus remove from his mind the suspicion through which he had vehemently persecuted the holy David, thinking him his enemy. Therefore he was much afraid lest he should be accused of violating so great a mystery in Saul, because he had thus meddled even his clothes. For thus it is written: "And David's heart smote him because he had taken away the skirt of his cloak." But to the men with him, who advised him to destroy Saul thus delivered up into his hands, he saith, "The Lord forbid that I should do this thing to my lord, the Lord's christ, to lay my hand upon him, because he is the Lord's christ." Therefore he showed so great reverence to this shadow of what was to come, not for its own sake, but for the sake of what it prefigured.

You have grounds for complaint against the leaders of our people, since you are redeeming those whom they very often permitted to be taken captive or even themselves reduced to servitude. Scripture gives us an example of singular praise when it extols David to the very skies because, having his enemy Saul in his power, he spared him, cutting off but the hem of his robe as evidence of both the opportunity that was his and of his loyalty. Good God, how munificently will You reward the deed of this man who now negotiates for the liberation of so many oppressed souls, you who have exalted David for sparing the life of a single man! - "Life of St. Epiphanius"

After these things, David's heart struck him, etc. The Lord groaned for the human race, from which he took flesh, with the affection of compassion, because by its merits he was compelled to take away some part of its royal ornament, preferring that it might have the kingdom here and always in the future age, preserved by divine law. Finally, seeing the sinful city, he wept over it saying: For if you also had known, and the rest (Luke 19). He lamented with human pious compassion the detestable loss of the kingdom, which he decreed by divine justice would come to pass.