You're not wrong, THF. But Tom was a lawyer by training, not a Sicilian like Genco. As a lawyer, Tom would think logically about Carlo: he'd never betray Sonny because he'd be the prime suspect and would be wiped out immediately. End of story. Genco, as a Sicilian, would understand that after Sonny beat and humiliated Carlo, he'd thirst for revenge--and to hell with logic. I believe that when Tom thought to himself (in the novel) that "old Genco would have smelled a rat," he meant that Genco would have warned Sonny that Carlo would have to be watched very carefully for signs of revenge. If Sonny took that advice to heart and kept his temper in check, he might have seen that Carlo's second beating of Connie was intended as a death trap for him.