1 Kings 2:6 - Joseph Benson’s Commentary on the Old and New Testaments

Do therefore according to thy wisdom What in reason and justice thou seest to be fit. For though I was forced to forbear him, I never forgave him. Let not his hoar head go down to the grave in peace Punish him according to his demerits. “This dying order of David,” says Dr. Dodd, referring to Dr. Chandler, “was an order worthy of a good king, and fit to be given in the last moments of his life. The crimes which drew down this punishment upon Joab, have already been expatiated upon. Many reasons concurred to prevent David's calling him to an account; but it is plain he never forgot nor forgave his crime: nay, he could not, consistently with the law, have forgiven him if he had been inclined to it. His deferring his punishment so long, was no reason why he should always do it. Reasons of state prevented its being inflicted before, and reasons of state required its being put in execution at this juncture. In time of war it was dangerous to attempt it, on account of the power, influence, and military skill of Joab; in time of peace it was safe, because Joab's power was then upon the decline, and his services were unnecessary. Joab was ambitions, enterprising, and restless, and having not proved very loyal to the father, might have practised the same perfidy against the son; who being young, and scarcely settled in his throne, might have suffered from his treachery, his want of fidelity, and his ambitions views, which were insatiable.”