What made the narrator confess his crime in "The Tell-Tale Heart"?
"The Tell-Tale Heart" by Edgar Allen Poe of course features an unreliable first person narrator who commits a crime that he feels compelled to confess at the end of the tale.
We are presented with a narrator who, in his own words, has suffered a disease which had sharpened his senses:
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.
Of course, the narrator's inability to tell the story "calmly" suggests his lunacy. However, also note the way that this heightened sense of hearing actually is what drives the narrator to confess his crime. After killing the old man and stowing his body away under the floorboards, the police come, and although the narrator tells us that he managed to convince them that he had no involvement in the disappearance of the old man, he begins to hear something that he cannot ignore:
Yet the sounds increased--and what could I do? It was a low, dull, quick sound--much such a sound as a watch makes when enveloped in cotton.
It is hearing the "tell-tale heart" because of his acute hearing that forces the narrator to confess his deed as he remains unable to ignore the loudening sound of his own guilt and crime. Perhaps we can infer that what he is symbolically hearing is his own conscience, forcing him to face the consequences of his actions, as it is only the narrator that hears this sound. Either way, it is this sound that forces him to confess, shouting, "It is the beating of his hideous heart!"