Mental Hospital - Su-Mi (Spoiler)

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2) I thought that was supposed to be after all that happened later in the movie. But I may be wrong.

This sentence has nothing to do with what I just have written above.

1 and 2: Her being in the hospital is the last thing that happens. The events in the movie all occur prior to that.

3. Yes, the stepmother was indirectly responsible for the deaths. We can assume she also feels some guilt later, as she sees Su-yeon's ghost near the end of the film. The people who see strange things in the film all suffer from mental trauma of various degrees. The film's director is clear about this in the DVD/Blu-ray audio commentary: the film is not so much about the supernatural, but mental illness. Su-mi suffers the most, and she sees things practically all the time. The stepmom and the sister-in-law (who sees a ghostly being under the kitchen sink) also feel the trauma but perhaps to a less extent.

4. The deleted scene titled "Poison" explains how Su-mi kills herself. She wants to poison the stepmom by injecting poison into her stepmom's medication. But the stepmom is HERSELF! So by taking the poisoned pills, she kills herself. The director's commentary says that subconsciously, the guilt-stricken Su-mi wants to punish herself.

5. I agree it is in bad taste to bring the stepmother's family members into the house when the girl's mother is still upstairs feeling distraught. We can assume the sister-in-law also feels some guilt, and may also even be a witness to the ensuing deaths on that fateful day. That may have traumatized her, resulting in her later convulsing on the floor and seeing a ghost under the kitchen sink.