2 Samuel 3:27

27So when Abner returned to Hebron, Joab took him aside to the middle of the gate to speak to him privately, and there he struck Abner in the abdomen so that he died, 3:27 This was an act of murder, not justifiable revenge, because Abner had killed Asahel in self-defense during a battle.to avenge the blood of Asahel, Joab’s brother.