I first saw this about 35 years ago, when I was 10. I haven't seen it again until a couple days ago.
I realize I remembered the ending wrong. But I want to tell about it because I think it's interesting.
The part at the end, when Niles is in the barn cellar talking to Holland. Niles says he knows who he (himself) is. Holland asks "Are you sure?"
I thought that meant that it was not Holland that died -- it was Niles. And that Holland had taken Niles' personality. My faulty recollection of the movie had me remembering Holland pushing Niles down the well.
That's how I remembered it until a couple days ago until I saw it again.
Thinking about it, though, I think that would be a neat double-twist ending.
(Spoilers) There appear to be three theories, each of which have their pros and cons. I'm agnostic about it, although I will say that when I first saw the film, in first release way back in 1972, I assumed it was a straight up supernatural possession story. In my defense, 1) I was only 12 years old and 2) there were a LOT of movies being made in that time frame that dealt with possession, reincarnation, etc. and I thought this was more of the same. To summarize the three main theories:
1) Holland is the evil twin. He possesses Niles from beyond the grave using The Game, and used Niles' body to continue committing his evil deeds.
2) Niles is crazy/evil. The death of his twin traumatized him so much that he develops MPD and commits evil deeds while under the influence of the "Holland" side of his personality. Or else he was evil all along, but is unable to admit it to himself, so has invented Holland to take the blame.
3) Holland is the evil twin, but he didn't die in the well. Instead, he pushed **Niles** down the well and assumed his identity so he could use the persona of the "good" twin to mask his evil deeds.
It's a testament to Tryon's tale that we are still 'guessing' about the central issue this far along. Any of those things could be true and he gives enough weight to all of them to make for a pretty deep plot.
Do you think #3 could be true even thought we watched Holland fall into the well while trying to throw the cat down it? Also, What was the cut lock at the very end meaning? Niles was under the room that burnt and lived???
The book clears #3 up (maybe...) The lock cut at the end (remember that the twins wanted to do that escape from the box trick by having the other magically appear at the back of the crowd, but the unconscious logic of 'one' of them told them that it would have to be a trap door.) I have heard the sound of Niles hack-sawing during an earlier scene. The editing works well, here. "Escape from the box". Tryon had a bizarre sense of humor on that.
SuCue- that is a very good summation of the possibilities, very succinct. Thank you, it will help a lot of people and perhaps you should post it as a separate post for discussion.