Zaf-the-don, I agree with you the most so far.

The fact that Michael in the beginning made plans to stay away from his family and chose someone like Kay to be with already shows that something inside of him was reaching for the "normal world" (thinking:"I won't be part of this, because it's wrong and I'm better than that"). And that "something" kept on causing that conflict within him; hence he keeps deluding himself that he's going to become legitimate ("once I become legitimate, all is fine, I am no longer a criminal, I don't have to feel bad, I will be redeemed" etc.). When you look at him, from the moment he kills Sollozzo and McCluskey, everything seems to be a burden for him. In a way it wasn't for Vito. Michael felt disdain for both - the crime organization (basically who he himself became)and "senators and governors". In his mind, one wasn't better than the other (don't be naive and think they both don't have men killed). When someone is deep down ashamed of himself and hates himself, he often blames the world for it. And everybody else suffers... especially when this person holds such tremendous power in his hands.
Michael was clearly not right for this "job". Because he wasn't able to separate business from personal... as in business from his own feelings. In order to "function" properly as the Don, he simply could not allow himself to feel. As a result, all these horrible things happened one after another. Because he had to be "strong for his family". That's what he believed, but he had become blind to the reality. This corrupt world was suited for the less sensitive and more balanced personality like Vito's, but it pushed Michael over the edge. (He should have become a mathematics professor, more "safe")

Michael's bravery and self-sacrificing nature (joining the army, later protecting his father) are the first things about him that we are introduced to in the first film. Before we are introduced to his Sicilian blood that eventually "wins out" (thirst for vengeance, etc). And this basic nature does not change during the events in part two when he's obviously on a power trip. It's very interesting how he's basically two opposite things at the same time - selfish and controlling yet does not care if he risks his life for others.

In Vito's case... need to survive is one thing, need for vengeance is another. Just because you "don't have the heart" to harm your family members does not make it ok to feel no qualms about killing other people who get in your way. Murder is murder.
For me, the most ruthless murder scene out of all three movies was when Vito practically sliced Don Ciccio open. He doesn't have the heart to kill Carlo himself, but he has the heart to leave that particular task to his most precious son.
I guess I see Michael as a mental institution material in the end and Vito simply as a prison material.

But, well... in the end The Godfather is just a dark modernised fairy tale.