2 Samuel 1:14

But when no laws or lawful authorities give command, it is not lawful to kill another, even if he wishes and asks for it and has no longer the strength to live, as is clearly proved by the Scripture in the book of Kings [Samuel], where King David ordered the slayer of King Saul to be put to death, although he said that he had been importuned by the wounded and half-dead king to kill him with one blow and to free his soul struggling with the fetters of the body and longing to be released from those torments.

We know that David went beyond the precepts of the law when, despite Moses' command to pay back one's enemies in kind, he not only did not do this but even embraced his persecutors in love, prayed devoutly to the Lord on their behalf, even wept mournfully for them and revenged them when they were slain.