Dark world Asriel is 100% gonna lean into being evil and Flowey like. My prediction is he is actually gonna be a great caring brother in the light world but in the dark world he is going to see it as fiction, a way to release his anger, and he is going to parody that Nerdy/pure boy who does the evil route in video games because he is too good of a person to do anything like that in real life.
That is assuming Toby doesn't cuck us and make Asriel not appear in any dark worlds.