Now if Mike had him killed because he felt that Fredo's STUPIDITY could be a threat to his life, then that's a different story. But after Fredo's outburst I believe that Michael took it as if Fredo INTENTIONALLY set him up and betrayed him knowing that an attempt was going to be made on his life and that is why he had Fredo killed.
I think that Fredo's stupidity is part of it. Fredo was stupid, envious and resentful, a very dangerous combination. I don't believe that Michael thought of his brother as a killer. I think that when he saw the depth of Fredo's anger in the boathouse, combined with how gullible he was, he knew that the combination could prove lethal. And that's why he had to have his brother eliminated before he could do harm again.
