I thought in the book that Tom was much more ruthless compared to his mild manners in the movie. I thought the movie tried to make him seem like he was legit and never got his hands dirty, but it seemed he was the main force behind setting Geary up in the novel. I think other than Michael, he would have been the best choice for Don even though he was not Sicilian.

Michael's fault was always revenge. Take for instance Rocco, that was a straight suicide mission and Tom was right in that they should have just let him be since the Feds were hot on his tail. I think after Tom went and suggested suicide to Frankie at the federal lockup that if that didn't work Michael would have assassinated him somehow which would have been crazy as well.