I agree, TB, and I think that's when Michael truly realized what a danger Fredo was. I think that when Michael first realized that Fredo was the traitor, he gave himself the "poor, dumb, weak Fredo, who was probably lied to..." speech (probably to protect himself from the hurt he was feeling). But when Fredo launched into his little tantrum in the boathouse, revealing the extent of his rage and resentment, Michael knew that Fredo may well have gone into the deal with Roth with his eyes wide open.