Not to mention that Michael didn't really have a choice. If his father was going to be saved Sollozzo had to die. And Michael was the only one who could get close enough to do it. No one was happy about it (least of all Michael), but that's the way it had to be. If Michael hadn't had the motivation that the Sollozzo assasination attempt gave him to join the family he might have ended up teaching math at Dartmouth.