2 Samuel 3:7
And Saul had a concubine, whose name was Rizpah, the daughter of Aiah: and Ish-bosheth said to Abner, Wherefore hast thou gone in unto my father’s concubine? And Saul had a concubine, whose name was Rizpah, the daughter of Aiah: and Ish-bosheth said to Abner, Wherefore hast thou gone in unto my father’s concubine?
Ishbosheth accused Abner of taking his father Saul's concubine Rizpah, and that accusation enraged Abner against him. Abner took the rebuke as proof that Ishbosheth was a weak and incompetent king unworthy of his loyalty. The quarrel became an act of treachery that overturned the throne which malice and ambition had set up. Abner deserted Ishbosheth and went over to David, offering to bring all the tribes of Israel with him to David's side. David accepted his proposals and sent him away honorably to carry out that transfer of the kingdom.
The honorable welcome given to Abner as a valiant and famed warrior stirred the jealousy of Joab, the commander of David's army. Joab already held a blood feud against Abner because Abner had killed his brother Asahel in the war between Israel and Judah. Seeing in Abner's coming both a chance for revenge and a way to remove a prospective rival, Joab waylaid him. There he murdered Abner to avenge his brother's death and to rid himself of a dangerous competitor for power.