She was mentioned earlier in the thread, but I thought I'd throw her name back into the ring as she's often overlooked as a horror writer:
Joyce Carol Oates.
I'd recommend checking out her short story collections for a taste of just how disturbing, gory, and frightening her work can be, especially the collections labelled "tales of mystery and suspense" (ie The Museum of Dr. Moses), although her standard literary collections always contain at least a few horrific zingers (ie Heat, which has "Family", a science fiction dystopian horror story).