I think there is a strong prima facie case that everything that has ever been written is horror...
But this aside, one of my favorite horror stories of all time has to be the Bible. Warning: spoilers below!
An entity of dubious moral integrity exists alone in The Outer Void. Unsatisfied with keeping his own company, He decides to generate other intelligent life-forms in the Void - He knows this is a cosmic sin, but He quickly changes the rules to salve His conscience.
He later feels that His creaions don't quite live up to His own impeccable standards, and so He goes gangsta on practically every living thing on earth. Satan was kind of like Charles Manson - very charismatic, and his preferred mode of operation was to convince other people to do his dirty work. God, on the other hand, likes to get hands on. From cover to cover, God kills hundreds of thousands of men, women, and children.
Consider this, taken from
www.stanford.edu
"A person with a borderline personality disorder often experiences a repetitive pattern of disorganization and instability in self-image, mood, behavior and close personal relationships. This can cause significant distress or impairment in friendships and work. A person with this disorder can often be bright and intelligent, and appear warm, friendly and competent. They sometimes can maintain this appearance for a number of years until their defense structure crumbles, usually around a stressful situation like the breakup of a romantic relationship or the death of a parent".
Or the death of an only child, perhaps....?
His worshippers, on the other hand, present as textbook examples of sexually abused children, meeting challenges to their values, beliefs or lifestyle responses equivalent to 'daddy chose me because I'm special'.
In true Ligottian style, everyone is totally screwed from the beginning; God made heaven practically impossible to get into just so He could watch his creations squirm trying to get there, and even those that do make it realise how crap it is, and that they're stuck there forever.
End of rant - don't take anything I say too seriously, because I was probably stoned when I said it.