Of course Oli and everyone else here who took that position are right: Sonny's uncontrollable temper and poor judgment sealed his fate. The slam-dunk certainty that Sonny, after hearing that Carlo beat Connie again, would rush out to her, in the middle of a war, unprotected, was the basis of Barzini's plot against him. It couldn't miss.

That's the reason I think Tom had some culpability in Sonny's death--his background and training made him unprepared for that eventuality. As a lawyer and a non-Italian uninvolved in the muscle end of the business, he was an excellent Family front man for dealing with the largely Irish and Jewish judges and politicians Vito needed in NY, and the Mormon officials in Nevada. But, as Michael said, he was not a wartime consigliere. Lawyers are trained to conciliation and negotiation, not in waging bloody wars in the streets. And, law is logical, orderly and rational--Tom wasn't on the lookout for the irrational need--compulsion--for vengeance that is so much a part of traditional Sicilian male "honor. Whatever else Carlo was or wasn't, he was a Sicilian. I'm sure Barzini had no difficulty recruiting him to beat up Connie again by telling him that it was a surefire plan to redeem his "honor" after Sonny humiliated him in the street. Tom wouldn't have seen it coming.

Tom admitted it to himself, in the novel: After learning of Sonny's murder he concludes that he "was no fit wartime consigliere...old Genco would have smelled a rat." Of course he would have. Tom didn't.