Translations
So Joab and his brother Abishai killed Abner, because Abner had killed their brother Asahel in the battle at Gibeon.
So Yo'av and Avishai his brother killed Avner, because he had struck down their brother Asa'el in battle at Giv'on.
So Joab and Abishai his brother slew Abner, because he had slain their brother Asahel at Gibeon in the battle.
(Joab and his brother Abishai murdered Abner because he had killed their brother Asahel in the battle at Gibeon.)

