The tone of this film is absolutely superb. I did not find the ending ambiguous at all though. John quite obviously killed Caleb by letting go of the rope. It is clear thanks to 3 scenes that follow: (1) John stands over the edge of cliff contemplating suicide. (2) Ann knocking the glass off the edge of the table. (3) John folding his hands in the same shape that Ann (and Caleb) did throughout the film right before praying. Why else would he suddenly need some sort of redemption?
Here is what I DID NOT like. -I read that the dog is killed in the book. In the film though, he just..disappears without any explanation. -Really though? First John pulls Caleb up from almost slipping and falling to his death once. Then Caleb is clearly standing sturdy ground. I watched this scene a few times over and it truly seems like he purposely fell again. I know that might sound crazy, but just watch the scene again. Almost like he was testing John. Then there is the long and poignant stare that John and Caleb give each other before the scene cuts.
Maybe Caleb let go. Just saying.