1 Kings 2:5 - Joseph Benson’s Commentary on the Old and New Testaments

Moreover, thou knowest, &c. After David had given Solomon this general charge, he proceeded to direct him to do some particular acts of justice and kindness, and first of all, bids him remember how Joab had acted. We must not look upon this admonition of David, on his death-bed, as proceeding from a spirit of private revenge, but as advising the execution of a public piece of justice which the circumstances of things would not permit him to inflict before. Certainly the punishment of Joab was owing both to God and man, for his treacherous and cruel murder of Abner and Amasa; and, therefore, David here, in justice to his people, and the divine laws, ordered his son and successor to do that, when his throne was fully established, which he could not execute himself for want of sufficient power, Joab's interest being then so great, that it might have thrown the state into a fresh civil war, had any steps been taken to inflict punishment upon him. What Joab the son of Zeruiah did to me That is, against me. For Joab's murder of Abner and Amasa was a great injury to David, as it was a breach of his laws and peace, a contempt of his person and government, a pernicious example to his subjects, and a great scandal to him, giving people reason to suspect that Joab had been only David's instrument, to effect what he secretly designed. And shed the blood of war in peace He slew them as if they had been in the state of war, when there was not only a cessation of arms, but also a treaty of peace. And put the blood of war upon his girdle This is added to denote his impenitence, that although by his perfidious manner of killing them, when he pretended to embrace them, he stained his own garments with their blood, yet he was not ashamed of it, but gloried in it, and marched boldly along with the army, with the same girdle and shoes which were sprinkled with their blood.