Albert Barnes Commentary 2 Samuel 20:11

"And there stood by him one of Joab`s young men, and said, He that favoreth Joab, and he that is for David, let him follow Joab." — 2 Samuel 20:11 (ASV)

He that favors Joab... - This speech, addressed to Amasa’s followers as well as Joab’s, shows very clearly that the rivalry between Joab and Amasa, and David’s intention to make Amasa captain in Joab’s place, were well known. It also reveals the real reason why Joab killed Amasa. The added phrase, and he that is for David, was intended to identify Joab’s cause with David’s and to insinuate that Amasa had not been loyal to David (see the note on 2 Samuel 20:5).