King James Version
So Joab and Abishai his brother slew Abner, because he had slain their brother Asahel at Gibeon in the battle.English Standard Version
So Joab and Abishai his brother killed Abner, because he had put their brother Asahel to death in the battle at Gibeon.New International Version
(Joab and his brother Abishai murdered Abner because he had killed their brother Asahel in the battle at Gibeon.)