I think in assessing Michael's life course, you also have to consider that while he always loved Vito, he grew emotionally much closer to him over the course of Part I--as we can see in the hospital scene where he says "I'm with ya now." Given the extremely traumatic events that take place over the course of the movie, it could hardly have been otherwise. As I said before, I think the best time for him to get out would have been when he returned from Sicily. But I think that, at least to some extent, his closer relationship with Vito played a huge role in his willingness to assume the family leadership when Vito became sick--particularly given the fact that Vito had no obvious successor. He wanted his father to have the peace of mind of knowing that the family was in secure and trustworthy hands. Furthermore, I think he realized that even if he refused to have any part in the family business, there would still be a Corleone crime family for the foreseeable future. While he certainly could have walked away from it, it's not as if he would have ended the criminal activities by doing so. I think that these factors, coupled with his desire for eventual "legitimacy," were what let him not to quit when it would have been easiest to do so.