Let not his hoar head go down to the grave in peace - It would have been an insult to justice not to have taken the life of Joab. David was culpable in delaying it so long; but probably the circumstances of his government would not admit of his doing it sooner. According to the law of God, Joab, having murdered Abner and Amasa, should die. And had not David commanded Solomon to perform this act of justice, he could not have died in the approbation of his Maker.

Do therefore according to thy wisdom,.... Which though young began to appear in him, even in the life of his father; he therefore exhorts him to use the wisdom he had, and take the first and fittest opportunity to cut him off for his former murders and late treason, as a dangerous man to his government and the peace of it:

and let not his hoary head go down to the grave in peace; that is, let him not die a natural, but a violent death; and let not his grey hairs be any argument for sparing him, or any reason for delaying the taking of him off, because he would in course die quickly; for he must be now an old man, as old as David, or perhaps older; since he had been his general forty years, even all the time of his reign; see Sa2 2:13.

Do . . . according to thy wisdom--Joab's immense popularity with the army required that any proceedings instituted against him should be taken with great prudence and deliberation.