I don’t know why you mentioned rayleigh’s Epithet but anyway. devil, demon, Yes that’s how he is described but what that’s gotta do with this, Asura feels like a devil fruit magic power, just because people see him as demon and describe as one he zoro developed a demon technique!!?well okay. Or you mean he is half human half demon?..or something else?..
Either way I don’t really care how he got this power, As long as explanation from ODA makes sense. Even if he doesn’t give an explanation, it’s fine but I really do want to know what this Asura is about, I have seen some good fan theories but they are only theories.