Human character is a complex thing that cannot change the way you please. It is developed by everything we encounter, but within it's inner laws. Among thousands students of that college it had to be Michael to head the Mafia. He was not just a random choice, he had it all in him already. He didn't know himself what his character was, until it showed itself. He could change, but in a logical way, consistent and true to his nature. Nobody ever changes without a cause, and killing people was not sufficient cause for such perversions of nature. It was all right with him, yet for you and me it would seem terrible. And btw, he was not the only person MW makes to act out of character. Healthy people do not become at once stupid if they were smart, etc. Puzo makes everyone in his book change, but wery subtly, more like revealing their hidden qualities. That's closer to reality.