2 Samuel 19:13 - Joseph Benson’s Commentary on the Old and New Testaments

Say to Amasa, Art thou not of my bone, &c. That is, nearly related to me, being my sister's son. God do so to me, and more also, &c. He solemnly promises to prefer him to the highest command in the kingdom; for he now thought it a fit time to depress Joab, who was grown insufferably insolent and imperious, and who, through his credit with the army, had protected himself in the commission of the greatest crimes. He had slain Abner most perfidiously in cold blood, and killed Absalom contrary to the king's express command, and now lately had insulted him in his sorrow. Having, therefore, now an opportunity of transferring the command to another person, who had as great an interest in the people's favour as Joab, he gladly embraces it, that so he might both chastise Joab for his faults, and rescue himself from the bondage in which that general had hitherto held him. Some, however, have thought that, considering Joab's very faithful services to David in all the changes of his fortune, and that his violent measures proceeded in part from a regard to him, as judging them necessary for his safety and tranquillity, David's conduct in this instance, in making Amasa captain of the host in Joab's room, is not an amiable trait in his character, and was not a prudent step at this time, especially considering Joab's violent temper: and, it must be acknowledged, it brought on the murder of Amasa.