Michael had to kill Fredo because it was Business-kill or be killed. Once Fredo talked with Johnny Ola - that was it, he was out of the "family" - he knew that. In Cuba he told Michael how uspet and hurt he was because they weren't close enough and because Michael took care of him - MIchael says that's how pop wanted it - did Vito tell Fredo, that Michael would head the family? Was this truly a surprise? I also wonder, did Michael just not trust Fredo at all? Couldn't he have kept Fredo around the top instead of isolating him?