2 Samuel 3:30 - Joseph Benson’s Commentary on the Old and New Testaments

So Joab and Abishai his brother slew Abner For though Joab only committed the murder, yet Abishai was chargeable with the guilt of it, because it was done with his consent, counsel, and approbation. And he probably was near at hand when the bloody deed was perpetrated, ready to assist Joab if there should be need. Because he had slain their brother Asahel in battle Which he did for his own necessary defence, and therefore it was no justification of this treacherous murder.