TB, I disagree with your interpretation of Michael's confession. While he did say that he had Fredo killed because he had injured him, I don't think he was trying to justify the murder to Lamberto. Instead, I think he was trying to impress upon the Cardinal how deep his transgression was - that he had killed his father's son for such a trivial reason. I think he was trying to impress upon the Cardinal how beyond redemption, what a terrible sinner, he truly was.