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 is one of the occasions where the novel and film differ, in my opinion. In Part II, we see Genco almost literally wet himself when Fanucci gives him the eye. Vito, of course, saw through him immediately. So much so that he risks incurring his wrath by offering him much less than he demanded from the clothing heist. Of course there was no wrath because Fanucci was all facade. So would the movie's Genco have smelled a rat? Doubtful, given what we were shown of his judgement?
