Albert Barnes Commentary 2 Samuel 3:24

"Then Joab came to the king, and said, What hast thou done? behold, Abner came unto thee; why is it that thou hast sent him away, and he is quite gone?" — 2 Samuel 3:24 (ASV)

Joab saw that if Abner reconciled with David, his own position as second-in-command would be forfeited; and so, with characteristic unscrupulousness, he proceeded to take Abner’s life.