2 Samuel 3:6
And it came to pass, while there was war between the house of Saul and the house of David, that Abner made himself strong for the house of Saul. And it came to pass, while there was war between the house of Saul and the house of David, that Abner made himself strong for the house of Saul.
While the house of Saul and the house of David were at war, Abner made himself strong for Saul's house by sustaining a throne that had been set up through malice and ambition until treachery brought it down. He had been the mainstay of that rival kingdom, holding it in place by his own strength and influence through the prolonged conflict. That throne endured only so long as his support upheld it, and its fall came when treachery overthrew what ambition had erected.
Becoming incensed against Ishbosheth, whom he regarded as weak and incompetent, Abner deserted his cause and went over to David with the offer to bring all the tribes of Israel to him. David accepted his proposals and sent him away with honor to carry out the transfer of the tribes. The favorable reception given to so valiant and famed a warrior stirred the jealousy of Joab, the commander-in-chief of David's army. Because Abner had killed Joab's brother Asahel in the war between Israel and Judah, Joab seized the chance to avenge his brother's death and to remove a prospective rival by waylaying and murdering Abner.