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.
Related Commentaries of 2 Samuel 3:28
2 Samuel 3:28
28 And afterward when David heard it, he said, I and my kingdom are guiltless before the LORD for ever from the bloodf of Abner the son of Ner: