After leaving David, Joab sent messengers after Abner. They brought him back from the cistern of Sirah without David knowing about it.
Certainly you must know that Ner’s son Abner came to deceive you, to find out about your movements and learn everything you’re doing!”
When Abner returned to Hebron, Joab took him aside in the gateway as if to talk to him privately. There he stabbed Abner in the belly. Abner died because he spilled the blood of Joab’s brother Asahel.