וְיוֹאָב֙ וַאֲבִישַׁ֣י אָחִ֔יו הָרְג֖וּ לְאַבְנֵ֑ר עַל֩ אֲשֶׁ֨ר הֵמִ֜ית אֶת־עֲשָׂהאֵ֧ל אֲחִיהֶ֛ם בְּגִבְע֖וֹן בַּמִּלְחָמָֽה׃ (פ)
So Joab and Abishai his brother slew Abner, because he had killed their brother Asahel at Gibeon in the battle.
Read halakhic rulings (Jewish law) on II Samuel 3:30: and classical halakhic sources.