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

So Joab and Abisai his brother slew Abner, because he had killed their brother Asael at Gabaon in the battle.

And Joab and Abishai his brother slew Abner because that he put to death Asahel their brother, in Gibeon, in battle.

So Joab and Abishai his brother put Abner to death, because he had put to death their brother Asahel in the fight at Gibeon.

And so, Joab and his brother Abishai killed Abner, because he had killed their brother Asahel at Gibeon, during the battle.

So Joab and (l) Abishai his brother slew Abner, because he had slain their brother Asahel at Gibeon in the battle.
(l) Abishai is said to slay him with Joab, because he consented to the murder.

So Joab and Abishai his brother slew Abner,.... For though it was only Joab that gave him the thrust of which he died, Abishai was in the secret, and was consenting and advising to his death, and so an accessory to it; and might be present, ready to assist in it, if occasion required; and so Josephus (k) expressly says, that Abishai his brother was with him when he drew his sword, and smote him:
because he had slain their brother Asahel at Gibeon in the battle: which was no just reason at all; his blood was shed in war, this in peace, in cold blood, and under the guise of friendship; that was shed with reluctance, and after fair warning, and in defence of himself; but this wilfully in Joab, unawares to Abner, and in great deceit and hypocrisy; see 1-Kings 2:5.
(k) Ut supra. (Antiqu. l. 7. c. 1. sect. 6.)

Abishai - For though Joab only committed the murder, yet Abishai was guilty of it, because it was done with his consent, and counsel, and approbation. In battle - Which he did for his own necessary defence; and therefore it was no justification of this treacherous murder.