Here's a fun little game: what's the scariest thing you can imagine?
I've heard it said that apparently the scariest thing Stephen King can think of is to be cut into lots of tiny little pieces. That's more what I'd call "Terror" rather than "Horror": terror being a fear based on harm to the body, as in that bastard subgenre of horror movies, the "slasher" flick.
True horror on the other hand, involves a component of fear based on the consequences of harm to one's mind...or immortal soul.
This is the realm for example of H. P. Lovecraft, whose tales of the Cthulhu mythos revolved on stunning revelations about the true malign nature of the Universe, embodied in the existence of truly alien and unequivocally evil god-like beings who refrain from devouring us all only because they happen to be slumbering...for now. These facts in the tales tended to drive the discoverers insane.
A concise and humorous summation of Lovecraft's writing style can be found here.
Stephen King is even more concisely summarized here.
Oh, what is my scariest thought of all?
Well.
How about this.
Gather 'round.
Ready?
Ok.
Imagine that it turns out that there is self-conscious Life just teeming throughout the Universe.
Habitable planets are everywhere.
But wherever we go, we turn out to be the wisest, most advanced species of all.
By far.
Now, how does that grab you?
Would that not be the most horrible of all plausibly realistic Universes???
:-)
UPDATE: Of course, I write this AFTER the unspeakable horror of the possibility of "President Kerry" receded into oblivion...