Charles Ellicott Commentary 2 Samuel 3:9

"God do so to Abner, and more also, if, as Jehovah hath sworn to David, I do not even so to him;" — 2 Samuel 3:9 (ASV)

So do God to Abner. —The anger of Abner culminates in a solemn oath to transfer the kingdom to David, as the Lord hath sworn to him. There is no record of a Divine oath to give the kingdom to David, but the prophetic declaration that God’s choice of him was unalterable (1 Samuel 15:29) may well have been considered to have the force of an oath. Abner does not propose to do this in order to fulfill the Divine will, for his words show that he had been acting until now in conscious opposition to that will, but to revenge himself for the insult now offered him. He had doubtless also become convinced of his master’s entire unfitness for the throne, and his power over Israel opened before him the prospect of high advancement from David.