2 Samuel 3:23
When Joab and all the host that was with him were come, they told Joab, saying, Abner the son of Ner came to the king, and he hath sent him away, and he is gone in peace. When Joab and all the host that was with him were come, they told Joab, saying, Abner the son of Ner came to the king, and he hath sent him away, and he is gone in peace.
Joab returned with all his troops and was then informed that Abner son of Ner had come to the king and that the king had sent him away and he had gone in peace. Abner had grown incensed against Ishbosheth, who was weak and incompetent and whose throne had been established by malice and ambition. He had therefore deserted Ishbosheth and come over to David, offering to bring all the tribes of Israel over to him.
David accepted Abner's proposals and dismissed him with honor to accomplish the bringing over of the tribes. 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. A blood feud already stood between them, because Abner had slain Joab's brother Asahel during the war between Israel and Judah. Joab seized this opportunity both to avenge his brother's death and to rid himself of a rival, waylaying and murdering Abner.