John Gill 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, and more also
He wishes the worst of evils to himself, such as he cared not to name; but left them to be supposed what he meant as utter ruin and destruction of himself, soul and body:

except as the Lord has sworn to David, even so I do to him ;
meaning if he did not do to David that which God has sworn should be done, namely, what follows, the translation of the kingdom to him;

By which it appears that Abner knew of the promise and oath of God respecting this matter; and therefore acted against his conscience, in setting up Ishbosheth on the throne; which he knew would not prosper, and that he was fighting against God; which shows what a hardened wicked creature he was, and how far ambition, and being thought to be of consequence, will carry a man.