I watched GFII again on AMC last night, and thought about a lot of posts (mine included) around here lately which have been pretty harsh toward Michael for killing his brother. There are a couple of scenes that make me wonder if Mike's decision to kill Fredo was reached only after Fredo came back to Nevada.
In Cuba Michael calls out to Fredo and tells him to come with him because it's the only way out. He shouts, "You're still my brother!" The first thing he does when he gets to Vegas is he sends Rocco and Neri outside to ask Hagen about Fredo's whereabouts, and then tells Tom to get word to him that he can return to the compound, that he understands that he had been misled, etc.
After Michael and Tom figure out that Pentangeli is alive and that Michael has been set up in a perjury trap, Michael asks Tom what Fredo knows, and Tom says Fredo says he doesn't know anything. Michael calmly tells Tom he wants to meet privately with Fredo when the great "I'm smaht" scene takes place. During the scene Michael asks Fredo whether there is anything he knows that can be of help, and Fredo says he knows Pentangeli is alive, and then he adds that Questadt belongs to Roth. Seems to me that had Fredo told this to Tom and Michael before the hearings they would have devised some different approach to things, perhaps even outing or taking out Questadt. It also shows that Fredo's treachery against Michael was ongoing because he did not come clean when Michael gave him what apparently was another chance.

Fredo's statement enrages Michael and it is at that point IMHO that Michael decides that Fredo is an ongoing danger to his hold on The Family and that he neds to be eliminated. This also would cast Michael in a somewhat better light than many of us have been putting him.