1 Kings 2:5

In his directions with respect to certain important persons, David, anxious for the security of his young successor’s kingdom, allows old animosities to revive, and is willing to avenge himself indirectly and by deputy, though he had been withheld by certain scruples from taking vengeance in his own person. We must not expect Gospel morality from the saints of the Old Testament. They were only the best men of their several ages and nations. The maxim of “them of old time,” whether Jews or Gentiles, was “Love your friends and hate your enemies” (see Matthew 5:43); and David perhaps was not in this respect in advance of his age. Joab’s chief offence against David, besides his two murders, was no doubt his killing Absalom 2 Samuel 18:14. Another serious crime was his support of the treasonable attempt of Adonijah 1 Kings 1:7. But besides these flagrant misdemeanours, he seems to have offended David by a number of little acts. He was a constant thorn in his side. He treated him with scant respect, taking important steps without his orders 2 Samuel 3:26, remonstrating with him roughly and rudely 2 Samuel 2:24, almost betraying his secrets 2 Samuel 11:19, and, where he disliked the orders given him, disobeying them 1 Chronicles 21:6. David allowed his ascendancy, but he chafed against it, finding this “son of Zeruiah,” in particular “too hard” for him (margin reference).

Put the blood of war upon his girdle ... - Meaning “The blood of Amasa spirited all over the girdle to which the sword of Joab was attached; and the sandals on his feet were reddened with the stains left by the falling corpse.”