There is no question that both Vito and Micheal made a choice to be a criminal and to remain a criminal, but dont all criminals make that choice. My contention is that Mike choice to kill Sol and the police captain to protect his family, he knew that without killing them his father would remain a target and would be killed eventually. I think that had Sonny not been killed, Mike would not have stayed in the family business beyond killing Sol and the Captain. As a good sicilian he could not leave his family and the family business without a Corelone at the helm.