There was this creepy movie that came out years ago, One Hour Photo, with Robin Williams. Normally, I avoid horror movies like the plague. I (like most writers) have an over-active imagination and horror movies just set it off. I've literally made myself ill panicking while watching horror movies. For some reason, I didn't realize this was supposed to be a scary movie when I started watching it. Or maybe it was only scary to me. But it was this seemingly perfectly normal guy, the kind of guy you would smile at or say something nice to when you got your pictures developed, and never think of again. And then he went psycho.
But this concept of a seemingly normal human being "snapping" and turning into a monster is frightening. I mean if it can happen to a random person, can it happen to me? It's easier if you can point to something, blame something. If something caused a person to snap, then it can't happen to me. Right?
In Hoffman's The Sandman, the narrator starts off supposedly sane. At least, he sounds like a much more sane narrator than Poe's in The Telltale Heart, whose constant attempts to explain that he is not mad renders him quite mad. But for Nathanial, there is a cause, a childhood trauma. There's someone to blame for him losing his mind, for his obsession with Olympia. By the end of the story, we can guess that he was, in fact, quite insane from the jump, but Nathanial, at least, blames Coppelius, and later Coppola and his lens for his destruction.
In Barker's Dread, there's a clear villian, Quaid, who is purposely driving Steve insane. And again, there is a childhood trauma, being hit by the car and losing his hearing, which contributes. Again here, there's someone and something to blame for Steve's latter madness.
We don't have much information about Poe's narrator in The Telltale Heart. He mentions once his "disease" that has shparened his hearing and his senses. But we don't have much information about him, if he became mad through some cause or if he had simply always been mad, and only now snapped to smother the old man in his home. To me, this character is almost more frightening than either Steve or Nathaniel, because we can't explain away his maddness. We can't point to something and say "that caused it." Similarly with Quiad, we don't know much about him, beyond his terror of axe murderers and his sociopathic characteristics.
Perhaps having a cause for maddness gives the character back some of his humanity. We can empathize with trauma, with terrifying events out of their control. We can't empathize with Quiad, because we don't know why he is the way he is. We can't say, "Oh, it's not his fault, x...y...z...happened to him and made him this way." In the movie "One Hour Photo," it comes out at the end that Robin Williman's character had been forced into childhood pornography. We have somethign to empathize with -- it restores his humanity and we can no longer dismiss him as some crazy pychopath. To me, those characters we can't empathize with, for who's actions we can't come up with a reason or someone to blame, are infinately more terrifying.