Charles Ellicott Commentary 2 Samuel 3:28

"And afterward, when David heard it, he said, I and my kingdom are guiltless before Jehovah for ever of the blood of Abner the son of Ner:" — 2 Samuel 3:28 (ASV)

I and my kingdom are guiltless. —This was true. Joab’s act was entirely without David’s knowledge, and was not only against his will on moral grounds, but was in danger of proving disastrous to him politically; hence he takes the strongest means of showing his abhorrence of the deed.