Jamieson, Fausset and Brown on 2 Samuel 18:14

14. he took three darts ... and thrust them through the heart of Absalom--The deed, partially done by Joab, was completed by his bodyguard. Being a violation of the expressed wish, as well as of all the fond paternal feelings of David, it must have been deeply offensive to the king, nor was it ever forgotten (1Ki 2:5); and yet there is the strongest reason for believing that Joab, in doing it, was actuated by a sincere regard to the interests of David, both as a man and a monarch.