Then Joab went out from David, and he sent messengers after Abner, and they brought him back from Bor Hasirah, but David did not know it. When Abner returned to Hebron, Joab took him aside to the middle of the gate to speak with him in private. Then he struck him in the stomach there and he …

So Joab and Abishai, his brother, killed Abner because he had killed Asahel, their brother, at Gibeon in the battle.