Brief caveat to this post: I do not consider myself a horror writer. In fact, I have only ever written maybe two or three stories that had any horror elements within them whatsoever.
But as an audience member, I frequently experience horror. And artful storytelling has gifted me with enough experience to know that true fear arises from feeling as if we have no control over a situation. Why do you think a lot of modern-day horror stories have children as the main characters? Children have no control over virtually anything in their lives; throw a flesh-eating monster into the equation, and you’ve got yourself a scary situation.
My advice for anyone who wants to strike terror into the hearts of their audiences: don’t go straight for the “shock horror” (the gore, the violence, etc.). All you really have to do to make your audience feel fear is take away any sense of control from your characters, no matter the situation. And by sense of control, I particularly mean sense of choice or hope.
Are your characters walking down a dark tunnel? Have their only flashlight go out: now they have to go the rest of the way without the benefit of sight. Has someone in the group taken on the leadership role / revealed some kind of past experience with what is going on? Kill them off: now no one knows what to do.
Yes, monsters and gore are scary. But losing control of a situation is the root of all fear because it is something that could easily happen to us in reality; it is not a concept that that belongs purely within the realm of fantasy.