It may have to do with the fact that he was dragged into his role by circumstance as much as choice. He did not want to be a man like his father, but when his father was hot it became evident to him that unless he intervened his father and all he built would be gone. Had he not gone to the hospital that night, Vito would have been shot dead. Had he not kiled Sollozzzo and McCluskey Vito would be dead. He knew in his heart that Santino was not a good Don, and he sensed that Tom lacked the toughness to be a wartime consigliere. You can see the transformation from "nice college kid" to cold blooded killer as he sits in the chair hatching the plan to kill Sollozzo, it is a brilliant scene which ends with Michael telling Sonny "its not personal, its purely business."
As much as there is talk of a man having but one destiny, I do not believe Michael thought that was the case. Hebelieved he could be the master of his own desitny, and when that belief was chattered it transformed him.

In a way, Michael had the zeal of someone who converts from one religiion to another. Once he became a mafioso, he was going to be the meanest, toughest and coldest of them all. On top of that Michael was always something of a loner anyway. With that kind of personality, that kind of life change, and that kind of power, you have a lethal combination.