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.
Ibn Ezra
love to your fellowman as yourself Many people have said that the lamed is superfluous, like the lamed in “they killed Avner” [II Samuel 3:30] [literally: they killed to Avner]. In my opinion, however, it contributes to the verse’s meaning. One must love the good that pertains to one’s fellowman as much as one loves the good that pertains to oneself.