As the Lord hath sworn to David These words show clearly that Abner knew very well God had resolved to bestow the kingdom of Israel upon David; and yet he had hitherto opposed it with all his might, from a principle of ambition. That is, he had all this while fought against his own knowledge and conscience, and against God himself. Now, however, (but, alas! it is out of resentment to Ish-bosheth, and from a principle of revenge,) he complies with the divine will, and vows with an oath to do that to David which the Lord had sworn to him. Undoubtedly Abner talked most foolishly in this, as if God needed his help to bring to pass what he had sworn to David, or as if his opposition could prevent it!
Related Commentaries of 2 Samuel 3:9
2 Samuel 3:9
9 So do God to Abner, and more also, except, as the LORD hath sworn to David, even so I do to him;