Joab killed Abner to avenge the death of his brother Asahel (see 2 Samuel 2:18–23). In doing so he committed a sinister form of treachery as he used the name and goodwill of David to lure Abner back to Hebron and then do evil.1 Though he sought to avenge the death of Asahel, he likely also considered Abner to be a rival best gotten rid of.2 The fact that he killed him in Hebron, a city of refuge, only adds to the evil of his deed. Cities of refuge were meant to be a place where due process of law was followed (see Joshua 21:13; Numbers 35:24–25).3
27 And when Abner returned to Hebron, Joab took him aside into the midst of the gate to speak with him privately, and there he struck him in the stomach, so that he died, for the blood of Asahel his brother.