Why didn't David kill Joab himself instead of telling his son to do it for him?

[tonychanyt]

Before David died, he instructed his son in 1 Kings 2:

5 “Moreover, you also know what Joab the son of Zeruiah did to me, how he dealt with the two commanders of the armies of Israel, Abner the son of Ner, and Amasa the son of Jether, whom he killed, avenging in time of peace for blood that had been shed in war, and putting the blood of war on the belt around his waist and on the sandals on his feet. 6Act therefore according to your wisdom, but do not let his gray head go down to Sheol in peace.

Why didn't David kill Joab himself instead of telling his son to do it for him?

David and Joab went back a long way. Joab was a nephew of David (1 Chronicles 2:16). Joab fought against Ish-bosheth in 2 Sam 2:

12 Abner the son of Ner, and the servants of Ish-bosheth the son of Saul, went out from Mahanaim to Gibeon. 13 And Joab the son of Zeruiah and the servants of David went out and met them at the pool of Gibeon. And they sat down, the one on the one side of the pool, and the other on the other side of the pool.

Joab killed Uriah for David. David owed Joab personally. David used Joab for many tasks that Joab finished successfully. David had personal reasons to be grateful for Joab's past service.

Joab had some sway over some of David's soldiers. Killing Joab himself might cause potential fallout. Some soldiers might interpret that as David killing his loyal friend.

Joab had done many things for David's benefit, and to avoid triggering dissatisfaction from the military department, it was better that David did not kill Joab himself.

Solomon, however, did not personally owe Joab. He could order his execution based on Joab's unauthorized killing of Abner and Amasa. People would have understood that Joab had it coming. The new king would not allow that kind of subordination.