The way Murder Machine was written, you get the idea that the Gemini Twins are the sources for many of the tales therein. But even if that is the case, we don't know what the real truth is. Stories are often invented to protect the real killers, as murder has no statute of limitations. One thing is for certain, even if Casso orchestrated the hit, he could not kill a made Gambino without Big Paul's approval. So I think everyone can agree that Big Paul killed Roy Demeo, if nothing else. And that was Paul's style, something Sammy the Bull hated about him....he'd have people from other families kill Gambinos.