And when Abner was returned to Hebron, Joab took him aside in the gate to speak with him quietly, and smote him there under the fifth rib, that he died, for the blood of Asahel his brother. (2 Samuel 3:27)
A city gate in Oriental lands is a common meeting place. To carry out his purpose it was necessary for Joab to meet Abner before Abner reached David.
For the blood of Asahel.
Joab slew Abner for blood revenge. He may have justified his retaliation by the provision of Num. 35:26, 27. Interestingly enough, Hebron was a city of refuge (Joshua 20:7) and in view of that fact, Joab may have carried out his deed in the city gate. The death of Asahel, however, took place in battle, and the slaying was an unwilling and reluctant act of self-defense on the part of Abner. Joab may not have been familiar with these details. But he should have studied the far-reaching effect of his deed, delaying as it did the formation of a united kingdom for some time. So great was Abner’s confidence in David that he appears to have had no suspicions.