The boys are disturbed, and frightened, by their mom; when she returns home from cosmetic surgery, with her head almost completely wrapped in bandages. I'm pretty sure he died with his mom because the final scene is both boys reunited with their ideal mom. Kim Jee-woon originally wanted Jun Ji-hyun to play Su-mi, but she refused the role because she thought the script was too scary. The movie's quite disturbing, and very terrifying, throughout; but especially at the end. Goodnight Mommy takes more than it should to find its rhythm, but it eventually gains energy and becomes a very interesting psychological thriller.
As for the twist its nothing we haven't seen before, The Sixth Sense and The Others both use this twist we don't have to see it again.
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Then the evil twin Lukaz keeps trying to convince Elias to not believe her. Faced with such a dangerous adversary and her immobility, Eun-ah gathers four people marginalized by society, each with a different skill, to help her kill Jae-wook. This was so disappointing. The man can do no wrong in my eyes. The violence gives an impact similar to Michael Haneke's "Funny Games"with the cruelty of Elias with his mother. You are in for a real treat. Janghwa, Hongryeon ; lit.