Adam Clarke on 1 Kings 2:6

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.