One of the major themes in “The Tell-Tale Heart” is the effects of guilt or conscience and the descent into madness. In the story, the narrator’s sanity is definitely in question. He kills the old man because of his “evil eye” but then feels guilty about it.
The story depicts a rapid devolving of the narrator’s psyche. At first he is very proud of himself, and considers himself very clever to have gotten away with the murder. When the police arrive, he coolly tells them there is nothing wrong, then leads them into the old man’s room.
In the enthusiasm of my confidence, I brought chairs into the room, and desired them here to rest from their fatigues, while I myself, in the wild audacity of my perfect triumph, placed my own seat upon the very spot beneath which reposed the corpse of the victim. (p. 6)
However, his confidence gets the better of him. While he is in that room, his guilty conscience starts to bother him. He begins to imagine that the old man’s heart is still beating.
But, ere long, I felt myself getting pale and wished them gone. My head ached, and I fancied a ringing in my ears… (p. 6)
The ringing in his ears represents his conscience, and his growing mental instability.
It was a low, dull, quick sound—much such a sound as a watch makes when enveloped in cotton. I gasped for breath—and yet the officers heard it not. I talked more quickly—more vehemently; but the noise steadily increased. (p. 6)
The narrator begins to act more and more erratically, arguing “about trifles, in a high key and with violent gesticulations” (p. 6)
Ultimately, it becomes obvious that the narrator is losing his mind, and it might have been clear to the police all along—why else would they stay and talk about nothing?
“Villains!” I shrieked, “dissemble no more! I admit the deed!—tear up the planks! here, here!—It is the beating of his hideous heart!” (p. 6)
Quotes from: http://www.enotes.com/tell-tale-heart-text