How does Poe set the tone for the story in the first paragraph?

Poe liked to set a morbid fantastical tone early in his stories The Tell Tale Heart is no exception. The first few sentences give us the feeling that something horrible has been done by somebody with an unstable mind,

"TRUE! --nervous --very, very dreadfully nervous I had been and am; but why will you say that I am mad? The disease had sharpened my senses --not destroyed --not dulled them. Above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell. How, then, am I mad? Hearken! and observe how healthily --how calmly I can tell you the whole story. "

We read on because we are intrigued by the creepiness of it all.

the tell tale heart