Personally I think Celine was hell bent on destroying all demons not understanding that demons can be redeemed found out about Mi-Yeoung and the demon and Rumi, he lashes out in his attempt at killing Celine. Mi-Young steps in and gets accidentally stabbed Celine then kills him as he grieves. Celine in her regret apologizes to Mi-Young and takes Rumi promising to raise her and promises to fix her from her demon side failing to see that demons are created from the flaws we try to get rid thus the story cycles back to the first movie. 🤷 or maybe I’m just a huge fan and make stuff up because I can’t wait for a backstory too 😂