I have a slightly different take. Michael could have rejected the life at anytime, his decision to get into the life his alone. A family memeber never really ever has to take command of a family in the case of succession, so Mike's decision was his own. My view is that Mike's way of approaching command has always been kill all my enemies cus I can' bank on mercy. This was first executed in GF I and elaborated upon in GF II.

But you do make a good point I think him in and of itself may have turned a cold streak when he learned about the abortion. I know he was on the path and before the abortion, when he learned about the whereabouts of Fredo Michael said to Neri "I dont want anything to happen to him while mama is still alive".