2 Samuel 3:27
And when Abner was returned to Hebron, Joab took him aside in the gate to speak with him quietly, and smote him there under the fifth rib, that he died, for the blood of Asahel his brother. And when Abner was returned to Hebron, Joab took him aside in the gate to speak with him quietly, and smote him there under the fifth rib, that he died, for the blood of Asahel his brother.
Abner had broken with Ishbosheth and come to David with an offer to bring over all the tribes of Israel to him, and David had accepted him and sent him away in honor to accomplish that transfer of the kingdom. It was after that dismissal, upon Abner's return to Hebron, that Joab, who stood as commander-in-chief of David's army, waylaid him. Taking him aside in the gate as if to speak with him quietly, Joab struck him under the fifth rib so that he died. The act stands as base and treacherous murder, not honorable combat or lawful execution.
Joab was stirred by jealousy at the favorable reception granted to so valiant and famed a warrior. He was also driven by blood-feud, for Abner had slain his brother Asahel in the war between Israel and Judah, and Joab sought to avenge his brother's death. Joined to that vengeance was ambition, a desire to rid himself of Abner as a prospective rival for place and power.