Albert Barnes 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)

In his final instructions regarding certain key individuals, David was anxious for the security of his young successor’s kingdom. He allowed old resentments to resurface and was willing to seek vengeance indirectly through a deputy, even though his own conscience had previously restrained him from taking revenge himself.

We must not expect Gospel morality from the saints of the Old Testament, for they were simply the most righteous people of their respective times and cultures. The maxim of “those in ancient times,” whether Jews or Gentiles, was to “love your friends and hate your enemies” . In this regard, David was perhaps not ahead of his time.

Besides his two murders, Joab’s chief offense against David was undoubtedly his killing of Absalom (2 Samuel 18:14). Another serious crime was his support for Adonijah’s treasonous plot (1 Kings 1:7). But beyond these blatant offenses, Joab seems to have provoked David through many smaller actions, becoming a constant thorn in his side. He treated the king with little respect, taking important steps without his orders (2 Samuel 3:26), arguing with him harshly and rudely (2 Samuel 2:24–25), almost betraying his secrets (2 Samuel 11:19–21), and disobeying commands he disliked (1 Chronicles 21:6). David tolerated Joab’s influence but bristled under it, finding this “son of Zeruiah” to be “too hard” for him.

Put the blood of war upon his girdle... This means that the blood of Amasa splattered all over the girdle to which Joab’s sword was attached, and the sandals on his feet were reddened with stains as the corpse fell.