I'll submit that Fredo represents the physical man in the novel. Feed him, sex him, and he's happy. Pointedly, he actually does pretty well for himself in the novel, as he's alive at the end and has a "legitimate" job. Coppola changes that Freddy into the first movie's Fredo, and endows him with his father's "sweet nature." Having assigned Fredo that role, Coppola seems to apply the rule of "nice guys finish last" to him, and just knocks him around for three hours.