If you write horror, you know that sometimes it can be fun. The set-up, misdirection, the monsters and mayhem, blood and gore. There is fun in horror writing and readers often share in that fun.
But the question to ask is: Do you want readers to have fun? Think about the over-the-top slapstick gore of Evil Dead II, the self-referential nature of the Scream series or the unintentional hilarity of the Friday the 13th films. (And I am going to use films because they are more well-known.) Is that what you’re trying to write? Maybe you just want to tell a story about blood and guts to make the reader feel queasy.
Or are you trying to horrify the reader? Think The Exorcist or American Psycho. Stories that make us squirm. True horror stories are those that leave a lasting impression of something being just plain wrong.
So how do you write horror?

Horror is a Moral Genre

At its heart, a good horror story challenges our morality. (And don’t worry, I’m not going to ride a morality high horse here. Neither does Rev. Jonathan Weyer in his blog post, which I highly recommend.) It’s not the blood and guts that horrify us, it’s that our sense of right and wrong is thrown off balance.
What makes Psycho unsettling is not (just) the spooky house, insane mother and mad-slasher elements. It’s matricide, suggestions of incest and the mother being so hung up on her son getting laid she’ll kill to prevent it.
In other words: sex. Considering when Psycho was released, it was shocking material.
But the motivation of the killer in Psycho can be explained. What drives:

  • Hannibal Lecter in Silence of the Lambs (let’s assume the subsequent books/movies never happened)
  • The Joker in The Dark Knight
  • The demon in The Exorcist
  • The xenomorphs in Alien and Aliens

All of these characters (and more) are motivated by something we can’t understand. It is not just a rejection of our morality, but a worldview so different we can’t comprehend it. And we can’t we predict it—that’s what makes is terrifying. How do you stop Hannibal Lecter from targeting you? Why did the demon chose Regan McNeil and not some other little girl?
So if you want to accomplish this same result, you as a writer must be horrified.

What Horrifies You?

Look at the news. What makes you squirm? Now ask yourself: Why? Likely, it’s something you can’t understand. Be it a sex killer in an American city or mass killings in the developing world, people do things that not only horrify us, but confound us because we can’t understand why.
To use this realization as a writer, you must develop characters who act in a consistent way, but follow rules we don’t understand. A stark-raving killer is one thing. Anton Chigurh is infinitely more terrifying.
More than that, you must be horrified by what you are writing. A story I wrote, “While Gabriel Slept” (available in the anthology Night Terrors from Blood Bound Books and from Cast Macabre in audio form) deals with a man contemplating killing an infant—one of the major taboos in horror writing. Though I don’t reveal if the character goes through with it, in an earlier draft the character suffocates the infant. It was a horrible scene to write, but it needed to be. And readers of the scene commented on how unsettling it was, but also necessary to see how far the character had fallen into insanity.
So if you are going to write horror where you want to horrify the reader, you must be horrified first. The writing process must be difficult and unsettling. If you can achieve that feeling while writing, your readers will pick up on it and be equally horrified.