Charles Ellicott Commentary 1 Kings 2:5

"Moreover thou knowest also what Joab the son of Zeruiah did unto me, even what he did to the two captains of the hosts of Israel, unto Abner the son of Ner, and unto Amasa the son of Jether, whom he slew, and shed the blood of war in peace, and put the blood of war upon his girdle that was about his loins, and in his shoes that were on his feet." — 1 Kings 2:5 (ASV)

What Joab ... did.—The charge concerning Joab has a certain righteousness in it. David could not—probably since Joab’s knowledge of his great crime, he did not dare—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, spurting 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 carried out by the duty prescribed by ancient law on the “avenger of blood.” David disclaims it (2 Samuel 3:28–29; 2 Samuel 3:37–39), 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, did not dare to blame it, and quietly acquiesced 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 remarkably true to nature, that the old King made no mention of the act for which, nevertheless, in all probability, he bore the strongest grudge against Joab—the reckless slaughter of Absalom against his own express commands and entreaties—and did not deign to allude to his recent treason, which probably had already embittered Solomon against him.