I've posted previously about this subject what I believe to be the Superman III syndrome. In that film I believe Superman splits off into three supermen none of which is like the original. That's true of Sonny, Fredo, and Michael. They each had some of Vito's characteristics, but none had them all in the stew that was Vito Corleone. Fredo was nice even loving; Sonny was fierce; Michael was intelligent. In one bundle, quite a personage; separately, Madonne!

By the way, in the novel Tom tells Kay on behalf of Michael that treachery can't be forgiven.