Know your direction. If you are trying to be unpredictable without having a clear direction for your horror novel, you are cutting your own lifeline. Suspense has to move the plot forward to be truly scary. If you do not know what the point of a scene is, then neither will your reader. The first step to having a direction for your novel is to outline the novel in some form. I am not big on long, drawn-out outlines that inhibit creative thought. I prefer a page or two of the meat of the story. Write it down in short-story shorthand form. The only person who needs to understand this quick outline is you.
Break down each scene or chapter in your mind before you start with this quick shorthand method. Ask yourself what you want to accomplish with this chapter, as well as how it will move the plot forward. Is it typical of several other books? If it is, how can you add a twist? Horror novels thrive on conflict, and a new twist can set yours apart. What does the reader expect to happen?
Be unpredictable, which can be a huge burst of creativity to you. When you decide to add a twist, come up with the conclusion to the twist before you start writing. Foreshadowing is fine, but leave an element of surprise with each scene.
It is far better to leave a scene with the knife coming down than with a dead minor character. A dead major character is good, too, since it leaves the reader wanting to know what will happen next. Each chapter should end with a bite of what is to come. To keep it unpredictable, that bite should have a number of avenues that have not been seen before. If you must reuse a plot, make sure that you change major parts of it to be different from the rest.
Read your own material and ask yourself if it goes in the direction the reader will expect. If it does, fix it by going in a different direction. For example, if the villain comes back to life after being shot repeatedly, and it is important to the story, find a variant way. Instead of the villain coming back to life, have his spirit come back instead. Maybe a twin brother who could explain some previously mentioned material could come on the scene to shock the reader. Again, you see why you must plot ahead. An evil twin can only work if you foreshadowed the fact by having the villain accomplish things no single person could have, for example. The bottom line is to be different, and that takes a bit of planning.