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.
I don't see it that way. I think Michael stepped up because he knew it had to be done to protect his father. It was going to be a one and done event.
He knew he would have to go away and leave his lover and it would be a while before he would be home.
When he killed you could tell on his face that he wasn't the killer type. He left and went to Itlay and almost became a normal life once again, Still guarded but a more normal style. Normal enough to fall in love and get married. Without any other family members there I may add.
It wasn't until Sonny is reported killed that he is pulled back into the life not by choice, but knowing there was no one else to step up with Sonny gone. His father once again needed him. His family needed him.
It was what changed him and then he started getting that wild mind of his working. And look what it did to him.
