Many folks think horror is just gory, bloody, violent and not at all to their taste. I can understand that since some books and movies are filled with meaningless violence and bloodshed used for shock value. That can make it pretty horrifying - but I doubt it's in the way the author or screenwriter actually intended. That kind of work is more shock and nausea than anything else.
However, true horror writing is about suspense, atmosphere, character emotion and how they've been developed. It offers plot twists and dialog consists of more than screams.
Let's face it, fans of the horror genre read that genre for heart-pounding chills, but in order for it to be more than blood, gore and splatter, the writer has to give his or her readers a great story as well.
Keep in mind that readers - all genres - are there for the ride. They're willing to suspend disbelief and their first hand experience of the real world (in fact there's a name for it - the reader's trance), but the writer has to give them a reason to read other than fountains of blood (that never got me to read anything).
So if you're of a mind to tackle writing in the horror genre remember you need to start off with a really great hook - give your readers that reason to read.
Pick up a few good (emphasis on 'good') horror novels and read the first paragraph or two. Hooked? Want to read more? What was the hook that got you? Some begin with a strange, perhaps spine-tingling occurrence; others start out in 'normal' world and bring in the horror elements so quietly you hardly know they're there until Wham! you get your socks knocked off. That's why Dean Koontz is one of my personal favorites. He's a master at slipping up on you from behind. It's sort of like starting a small snowball rolling down hill go discover it's an avalanche at the end.
As a writer you have to realize as well that though your characters may well be ghosts, ghouls, witches or werewolves and vampires, they still have to be possessed of characteristics with which the reader can identify. They don't all have to be lovable or heroic, they can be out and out jerks, but something for the reader to get a hold of is imperative. Give them emotions and human characteristics. Give them names, mannerisms, actions and a way of speaking that makes them stand out to your reader. Think about names: How would you name your 'hero' and what kind of names would you give to a vicious villain? And apply the same standard for horror as for any other genre: don't give similar sounding names to our characters. Don't confuse your readers. Don't make them puzzle so hard over who is who that it distracts them from your story.
Think about descriptions as well. Remember you're aiming for atmosphere, tension and suspense. Less is more. Dole out details that tell your reader something about the character, but don't go crazy and fill your book with them. Let your reader get into the skin of your characters and use their own imagination to fill in some of the details. Point them in the right direction and let go. You don't want to stop the flow of your novel. Keep in mind all of your senses and bring them into play. Smell, taste, sight, sound and feel. Think about them as they apply to your characters, then pluck out the best ones to build your horror tale.
Breath life into your characters by making them real. Let them make mistakes. Let them think inappropriate thoughts - like how fat his business partner has gotten since he started spending all his time behind a desk.
And think of the detail in your story. If you are indeed using werewolves as main characters, how do they look? If they're completely normal as human, what happens when they transform? Do they have enough time to shed their clothes? What happens to their shoes? Are they naked when they change back (one would think so - but you may have an original take). Whatever you create in your world you must make it believable. Let your reader identify with the world you create, then give it some fantastical twist.
And, needless to say, in addition to character and setting, you need a really great plot and conflict. You can't just string together 'horrifying' scenes and expect it ti be a readable book.
Read a lot in the horror genre if you want to write in it. Find exceptional writers to analyze their storytelling. Don't think it will be an 'easy sale' to just do what's been done. Stretch yourself, grow. Read articles on the craft and books by great authors. Then, when you dream up something that seemingly could only exist in nightmares, you'll be well equipped to create a great novel from it.
You might also want to check out the Horror Writers Association.
Enjoy those things that go bump in the night.