Abner’s move to David is most likely driven by politics and not by theology.1 Though he made mention of the Lord, his actions had more to do with the offence caused by Ish-bosheth’s accusation than a conviction that he was wrong to support Saul’s son as king. He may have also considered that Ish-bosheth was certain to lose the fight2
9 God do so to Abner and more also, if I do not accomplish for David what the LORD has sworn to him,