2 Samuel 3:22
And, behold, the servants of David and Joab came from pursuing a troop, and brought in a great spoil with them: but Abner was not with David in Hebron; for he had sent him away, and he was gone in peace. And, behold, the servants of David and Joab came from pursuing a troop, and brought in a great spoil with them: but Abner was not with David in Hebron; for he had sent him away, and he was gone in peace.
Abner broke with Ishbosheth after becoming incensed against him, for Ishbosheth was weak and incompetent and his throne had been established by malice and ambition, and Abner deserted to David offering to bring all the tribes of Israel over to him. David accepted that offer and sent Abner away with honor in peace to carry out his purpose of winning the tribes. Just then the servants of David and Joab came in from pursuing a troop and bringing great spoil with them, while Abner was no longer with David in Hebron and was left unprotected.
The favorable reception given to so valiant and famed a warrior stirred the jealousy of Joab, commander-in-chief of David's army, who regarded Abner as a prospective rival. Between them lay a blood feud, because Abner had slain Joab's brother Asahel during the war between Israel and Judah. Joab seized the opportunity to basely waylay and murder Abner, both to avenge his brother's death and to rid himself of a rival.