1 Kings 2:6

“ Do therefore according to your wisdom, and do not let his hoar head go down to Sheol in peace.”

He therefore advised Solomon to act wisely in accordance with the situation as he knew it and, as soon as he reckoned that he had acceptable grounds, to ensure that Joab was executed. He was not to allow him to reach old age, or die naturally (i.e. he was not to allow his hoar head go down to the grave world in ‘peace’, that is, in a state of wellbeing) for he was too dangerous an enemy to have around. He would need to be watched carefully and dealt with the moment he stepped out of line (an attitude that Joab himself had demonstrated towards others)

There is no good reason for doubting that David did actually give this advice. No one knew Joab like David did, and he was clearly fearful of what he knew Joab to be capable of, especially as, by siding with Adonijah without consulting the king, he had shown whose side he was on (that too had been a betrayal of David). And he wanted Solomon to know it as well. He was not going into details on the rights and wrongs of the matter. He was simply indicating what kind of a man Joab was. He wanted Solomon to be fully aware that Joab was a man of blood, and that now that he had revealed his hand as a supporter of Adonijah it could only act as a danger signal for Solomon. It indicated that Joab had no sense of loyalty towards Solomon, in contrast with his attitude towards David.

In view of Joab’s loyal, if somewhat stained, service to David these words of David might appear somewhat surprising. But we should note that David was not calling for his immediate execution. He was simply warning Solomon that here was a man who needed to be closely watched and despatched if and when (as he had no doubt that he would) he stepped out of line. For we must remember that Joab had been commander-in-chief of the hosts of Israel for many years, and still was (1 Kings 2:35), and was thus a man of great influence and power in the kingdom. He was thus capable of doing great harm. He was the kind of man who, if he did not feel a sense of total loyalty, would be an ever constant danger, able almost to stir up rebellion at will. That was why Solomon, while leaving him in his exalted position, was to be sure that he watched him carefully and acted decisively if he strayed out of line. David did not want Solomon to be think that because of the relationship that he himself had with Joab, he was a man to be trusted (in contrast with Benaiah).