[Wasn't Barzini responsible for Sonny's death? It's what Carlo stated before he was whacked, that he was confronted by Barzini. Plus the Don said it to, "... it was Barzini all along" after the commission meeting.
Yes, it was Barzini, as Carlo confessed. But Tatt was in it with Barzini.
Something I don't understand is why would Michael make such a bold move in eliminating the heads of the five families, to move to Nevada? What's the point in that? He had the city, and the cosa nostra, in the palm of his hands. I could understand expanding your influence to Nevada, but moving it entirely?
As Olivant said, it was designed to add more violence and drama to the story. In the novel, Michael didn't whack Stracchi and Cuneo, presumably because he didn't have to--they didn't actively conspire against him (though they took sides against him); and anyway, by eliminating Barzini, there was no question that Michael was the top dog.
