The real killer, explained.
It was the mother.
The first time I realized the movie was written with the mother as an alternative killer, was when the detective said her cooking smelled good, and she said she was cooking crabs. Then he asked if it was the spicy style. She said yes.
This was a clue, that despite her daughters or step-daughters just being tortured and murdered, she didn't care. She wasn't sad, on the contrary she was just fine, cooking a nice spicy recipe.
After that, she is showing the detectives something that would implicate her husband, throwing them off her scent. Later, when she was telling the detective what Marina told her, but said Marina couldn't tell him herself? How convenient. But this time she was implicating Quini.
She was the one taking the photos, she knew all about the lodge, because it was where Quini used to take her. That's her real motive; seething envy of a woman scorned.
Everyone else lied to protect the mother. The caretaker who lied and said Antonio was 'Sebastian', the photographer who lied and said Quini bought the film, hey were all protecting the mother. We even saw the lady in the cap, that was her.
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