Neither Tom nor anyone else could have cooled Sonny's temper permanently. But, I believe Tom could be blamed for not realizing that Carlo would lust for revenge after being publicly beaten and humiliated by Sonny. Tom's lawyerly training in logic and conciliation would have let him to believe that Carlo ultimately wouldn't bite the hand that fed him. If he were wearing his "Sicilian hat," he'd have recognized that, in matters of "honor," logic takes a backseat to vengeance. In the novel (as has been pointed out elsewhere), Tom concludes, on his own, that "old Genco would have smelled a rat."