I'd say that in Puzo's world, there was destiny at work. There's a couple of instances early in the book where Puzo says that Michael takes on an eerie resemblance to Vito. As the Family responds to the attack on Vito, Michael participates, and it slowly becomes clear
that his separation from the Family occurred because it's an archaic organization, not because he thinks it does wrong. As he proceeds through his time in Sicily and eventually returns to America of one mind with his father, it's as if the real Michael has emerged, instead of Michael being changed by circumstances.