I'm just going to paste my advice from
this thread. I think it's especially appropriate for a horror novel.
Here is the best writing advice I've ever gotten:
Give them what they want -- NOT what they expect.
If a plot goes as expected, it's predictable and boring. A plot needs to be unexpected to truly satisfy. But that doesn't mean you can hurl something at your readers from way out in left field. You have to give them what they want -- even if they didn't know they wanted it in the first place.
And giving them what they want doesn't always mean ending happily. Look at Atonement. It shatters its readers in a completely unexpected way, but it's done so well, and lingers so long, the book is exactly how it should be.
We need twists and turns, minor and major, to keep books interesting (why read the same plot over and over?). But they need to fit with the tone, the voice, the characters and the plot. And they need to satisfy -- even if they devastate at the same time.