This is a better ending. Imma leave it here and fantasize that Ah actually does manage to keep them both in their own little damnation. Ah doesn't deserve it but istg he's the only one who could possibly manage to tame that demon.
Are we certain Seo isn't legitimately a demon in disguise my guys? He somehow didn't die after being tossed into an ocean, he hypnotized a man, legitimately brainwashing them for a good day or so, and somehow keeps completely duping everyone he meets.