1 Kings 2:6

Do therefore. Joab had murdered Abner (2 Sam. 3:27-30). David, at the time, made it clear he had had no part in the crime, and announced that in due time the Lord would “reward the doer of evil according to his wickedness” (2 Sam. 3:31-39). Joab had also murdered Amasa, whom David had just appointed to take Joab’s place (2 Sam. 19:13; 20:8-10). The death of these two was to be avenged. At the time these crimes were committed David was not in a position to punish Joab, owing to Joab’s knowledge of David’s part in the death of Uriah the Hittite (2 Sam. 11:14-25). But the dictates of justice demanded that such crimes as Joab had committed should not go unpunished. Therefore, David, speaking not as a private individual who had received many years of hard and faithful service from the man he now condemned, but as a theocratic king, gave directions that Joab’s crimes should be punished by a man whose hands were clean and who owed Joab nothing. It should be remembered, further, that Joab was guilty also of acts that David does not here expressly mention, such as the slaying of Absalom against David’s express command (2 Sam. 18:14, 15), and his recent treason in the support of Adonijah (1 Kings 1:7), which no doubt had already embittered Solomon against him.