There's no transition between Michael I (american hero, away of family business) and Michael II (Sicilian mobster). The shooting of Vito and the assassination of Sonny maybe explains a psychological turn to revenge (a little material showing the attachment between Michael and each of them would have been nice), but it's not enough for this transformation into a full-fledged mafia manager, interested in brothels and casinos. The character of Kay is pathetic, living among criminals and believing Michael's promises about changing his way of life. The whole movie is a collection of excuses to present sketches of violence.