1. 100% sure that the protagonist is insane, and denying another explanation should be based on the view that the movie was shot from the perspective of a lunatic/mentally ill, and finally the lunatic's mental illness was revealed under the guidance of a humanitarian psychiatrist . Of course, if we look carefully, we can find a lot of inconsistencies in the film, before the madman discovers his own truth - because this is the madman's point of view - and confirms that the protagonist is indeed insane.
2. There may be an explanation; because of my ignorance of film theory I'm not sure how to express it, but roughly the whole story is retrospective, that is, "memories": those that make the protagonist confirm that he is a lunatic , something that exists in his mind. If we can suspect that part of the story is the hallucination of a lunatic, why can't we suspect that the whole story is the hallucination of a once-normal person. (It's certainly not a falsehood to say it's an illusion; it's like a good lie is always mixed with "truth.")
3. Coup d'état: Hehe, I'm not saying this movie is about a coup, but if According to the "conspiracy theory" explanation, there is a bit of a coup d'etat. Perhaps because of this reason, the director only has a warm humanitarian psychology concern...
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