International Standard Version He said this because Joab and his brother Abishai murdered Abner after he had killed their brother Asahel in the battle at Gibeon.
American Standard Version So Joab and Abishai his brother slew Abner, because he had killed their brother Asahel at Gibeon in the battle.
Young's Literal Translation And Joab and Abishai his brother slew Abner because that he put to death Asahel their brother, in Gibeon, in battle.