1 Kings 2:5

What Joab... did. — The charge as to Joab has a certain righteousness in it. David could not — probably since Joab’s knowledge of his great crime, he dared not — punish him as he deserved. There is a graphic vividness in the description of the blood of his victims, shed as “the blood of war in peace,” spirting over the girdle and sandals of the murderer, which shows how the horror of the crimes had dwelt on David’s imagination. The murder of Abner, treacherous as it was, probably had some show of justification in the rough justice wrought out by the duty laid in ancient law on the “avenger of blood.” David disclaims it (2 Samuel 3:28; 2 Samuel 3:37), without actually condemning it as inexcusable. The more recent and shameful murder of Amasa was simply one of revenge and ambition, because Amasa had been put in Joab’s place; yet David, broken in spirit, does not dare to blame it, and quietly acquiesces in the resumption by Joab of the dignity conferred on the murdered man. That these crimes should be punished by a king whose hands were clean, and who owed Joab nothing. was perhaps just, certainly within the letter of the law; though clemency might have spared the old and now fallen warrior, who had at least served David ably with long and faithful service. It is singularly true to nature, that the old King makes no mention of the act for which nevertheless, in all probability, he most bore grudge against Joab — the reckless slaughter of Absalom against his own express commands and entreaties — and does not deign to allude to his recent treason, which probably had already embittered Solomon against him.