Matthew Poole and continuators on 2 Samuel 3:30

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

For though Ioab onely committed the Murder, yet Abishai was guilty of it, because it was done with his consent, and counsel, and help, and approbation: for by these and such like actions men are involved in the guilt of other mens sins, at least in Gods judgment.

Which he did in the fury of Battel, and for his own necessary defence; and therefore it was no justification of this unnecessary and treacherous Murder in a time of peace.

Marginal references: Chap. 2. 23.