If not take over the whole Commission, certainly dominate it. The novel makes clear that Barzini, younger than the other Dons, saw himself as Number 2, waiting in the wings for Vito to fold or die. He had already had Sonny whacked, and having Michael killed would leave Vito without heirs. He probably figured Vito would simply lose heart and fold, putting himself in the top position, Carlo would be a marked man--once a traitor, always a traitor.