In the novel, Tom blamed himself for Sonny's death. "He knew, now, that he was no wartime consigliere. Old Genco would have smelled a rat." This not to say that Michael should have blamed Tom for the Corleones' misfortunes--Michael was where the buck stopped. But, just consider: Tom didn't know that Pentangeli had survived the garotting and was in Federal custody, to be used as a witness against him. "Our people in the NY Detectives say he was half-dead, scared, talking out loud about how you betrayed him...Roth played this beautifully." Duh, Tom: how come you didn't know he survived if "our people in NY" did? He also tells Michael, "Fredo says he knows nothing, and I believe him." But, Fredo knew about Pentangeli's survival--and that the Senate lawyer, Questadt, belonged to Roth. Had he known that, and IMO he should have, he'd have protected Michael from that near-perjury rap.