"The Pit and the Pendulum" is certainly a "scary story." It's meant to give us goose bumps, make us shiver, and generally feel like a scaredy cat. That said, even as it taps into our emotions, it forces us to wonder about why and what we fear. In the story, something as seemingly inconsequential and mundane as thirst strikes fear into the narrator. As we read, we can't help but think, "What would I do in this situation?" And the most frightening thing of all may be the answer to that question. So remember, when you're reading a story, it's not only about how the characters feel, it's about how we feel, too, as readers. In this case: totally and completely freaked out.
Poe himself is sort of an inquisitor: after all, he's trying to scare the crap out of us.
Strangely enough, it's not the grotesque horrors that scare our narrator the most; instead, it's the possibility of entering into a state of nothingness.