Michael didn't WANT out!! He was IN, for life. He held his father's hand, and he said, I'm with you now, Pop. When the family was threatened, the "real" Michael surfaced, and it was a Michael that wanted revenge, no matter the price. As Vito says frequently in the novel, Every man has one destiny. This was Michael's. That destiny was cemented with the deaths of Sonny and Appolonia. He could've backed out when he came home, let Tessio and Clemenza take over the family, but he didn't. He wanted to take charge, plot his revenge, and then grow his source of power.

As for being legitimate, he couldn't. Even after his attempts at justifying his choices in GF3, what does he do? Kneeling at Don Tomassino's coffin after his confession, he swears that if he has one more chance, he will sin no more. And then he promptly rises, turns, and gives Vincent the order. And he paid the ultimate price.