2 Samuel 3:30
So Joab and Abishai his brother slew Abner, because he had slain their brother Asahel at Gibeon in the battle.
Cross-references
In 2 Samuel 2:19-23, Abner kills Asahel, Joab's brother — this event directly motivates the revenge killing of Abner in 3:30.
In 2 Samuel 19:13, David appoints Amasa over Joab, partly due to Joab's murder of Abner — a political consequence of that killing.
In 2 Samuel 20:8, Joab treacherously kills Amasa, mirroring his murder of Abner — a pattern of violent, deceptive killings by Joab.
In 2 Samuel 16:9, Abishai again offers to kill (Shimei), showing his impulsive violence — parallel to his role in killing Abner.
Proverbs 28:17 says a murderer will be a fugitive — directly applicable to Joab's bloodguilt from killing Abner.
In 1 Chronicles 11:26, Asahel is listed as Joab's brother — this identifies the family relationship that motivated the revenge.