Charles Ellicott Commentary 2 Samuel 3:27

"And when Abner was returned to Hebron, Joab took him aside into the midst of the gate to speak with him quietly, and smote him there in the body, so that he died, for the blood of Asahel his brother." — 2 Samuel 3:27 (ASV)

Aside in the gate. —The gateway was a customary place of conference in the East, and Joab there awaited Abner’s return; he then took him “aside” to some place of privacy, as the Septuagint reads, “by the side of the gate.” On the phrase “fifth rib,” see Note on 2 Samuel 2:23. The reason for this cold-blooded and treacherous murder on the part of Joab is expressly said to be for the blood of Asahel his brother; but no doubt his revenge was intensified by jealousy.