This movie is really slow, that's a fact, I almost stop paying attention twice: you can't deny that sometimes its quite boring. However if you manage to keep interests on the behavior and the story between each of the protagonist, you will just find the ending kind of mind blowing.

This theory is not from me, however I share it because it made a lot of sense if like me, you had this feelings that something was off about this ending, like something was not quite right... I don't agree with all that theory, however it might help you to figure out your own interpretation/theory...

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John did not killed Caleb, he saved him once and save him again when Caleb tried to fall on purpose. He wanted to test how good John was, Caleb did a creepy smirk the second time John was trying to pull him and that means that he understand that John was trustworthy and more deserving than himself. Because if John had let Caleb fall or pushed him, it would have almost most definitely damaged the watermill, and it looks unscathed at the end...

After that, they both moved the piano and then Caleb left, maybe because John convinced him that he wasn't good for Ann or maybe as a reward for saving his life, twice... But then, John was heartbroken after Ann went out to find Caleb, so he went to the fountain and jumped. Ann saw him and was sad from what happened (she pushed the glass from the table part).

Finally, it was John's soul watching Ann playing the piano. John looked happy, Caleb unknown, Ann lonely for being alone again.