I agree FS, but something happened to Michael after the incident with McCluskey. He told Vito he was now "with him" which of course was both a physical fact and a metaphor, but then the following day I think he saw the way Sonny and Tom were running things, and he realized they both didn't get it. Thats when he announced the plan to "kill them both." After everyone had a god laugh, he took apart Sonny's argument and then Ton's argument, and he found an ally in Clemenza (the smart one) who said "Mikey's right." He seemed a transformed man who knoew he was destined to lead the family.