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.”
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"
In the same way, Saul’s kingdom foreshadowed one that should last forever, though he personally was reprobate and rejected. The very oil with which he was anointed (the chrism by token of which he was called a “Christ”) must be understood symbolically as pointing to a profound mystery. David himself so religiously respected this anointed state that he was conscience-stricken when, in a dark cave where Saul had entered to ease himself, David came up, unseen, from behind and cut off a tiny piece of Saul’s robe. David did this merely to have evidence later how he had spared Saul when he could have killed him, thus hoping to disabuse Saul of the idea which drove him implacably to pursue David as his foe. Nevertheless, David quaked with fear that perhaps merely by so touching Saul’s garments he was guilty of sacrilege.… Such deep religious reverence was paid to this foreshadowing figure, not for what it was in itself but precisely because of the reality it typified. - "City of God 17.6"
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.
[AD 395] Gregory of Nyssa on 1 Samuel 24:4-5