1 Kings 2:6

Do therefore according to thy wisdom Compare 1 Kings 2:9. The wisdom to which David in both cases alludes is a politic method of bringing these men to do something for which they might be put to death. Such men were specially dangerous in the court of a young king.

his hoar head Most likely Joab was already an old man not much younger than David himself. This may account for some of the freedom of his speech to the king.

go downto the grave in peace The advice is put in a more direct form in 1 Kings 2:9, concerning Shimei, -Bring his hoar head down to the grave with blood."

The Hebrew construction (the jussive after לא not אל) seems to imply a suggestion rather than a direct command. -I recommend you not to let," &c.