What finally tore it for Michael was Fredo's outburst in the boathouse ("I was passed over," etc etc), but worse than that, he also feeds Michael a line of BS about the Senate committee and Questadt (which Michael KNOWS is BS), at that point Michael decides Fredo is still trying to snow him, and he has to go.
Fredo said, "The got Pentangeli," and "That Senate lawyer, Questadt, belongs to Roth." What was BS about that?
Nothing's BS about it, but it's more or less common knowledge, and Michael wants Fredo to tell him something he doesn't already know.
