Hm, I kind of agree with some of the things said in here, but not totally.

I see Vito's hypocrisy as much worse than Michael's.

It's just interesting how the word "cold" is used in reference to Michael's character. In my opinion he seems quite the opposite even. Tom Hagen is quite a "cold" character, not Michael. Michael is too passionate and sensitive for his own good. Sensitive people tend to have more sensitive egos as well.
When his own brother betrays him, he's so hurt he's unable to forgive ("You broke my heart!... You're nothing to me now.").
When he's furious, he's too furious to forgive. When he loves, he loves so deeply he's ready to sacrifice himself for those people. Which is shown when he's standing on the hospital steps to guard his father, while risking his life... and when he's shielding Kay with his own body to protect her from the bullets in part two. All of this seems to be instinctive.

I'd say him growing "cold" is not exactly the essence of his nature but more like a consequence... it's due to the constant conflict within him (between his "dark side" and his conscience). Vito didn't have that conflict, that's why he is warmer as well.. not stressed out. Nothing to feel guilty about (even though he should have).

In any case, Michael is very complex... at least Al Pacino makes him so on screen. In all three movies

I agree that our choices make our lives. But our choices are already predetermined by our genes. So the story seems to suggest that these men were destined to become criminals and murderers, because it's in their blood. Which leaves you with a tremendous feel of loss and tragedy. Especially in Michael's case, because he had such a promising future.