The moral of "The Tell-Tale Heart" is that we should not commit crimes because, in the end, our own sense of guilt will expose us.
In this story, the narrator takes cares of an elderly man but grows to fear and loathe what he calls his "Evil Eye." He becomes obsessed with it and decides to murder the old man. He does so and the man screams as he is attacked, but the narrator succeeds and buries the body under the floorboards.
The police come the next day, having heard reports of a "shriek" and wondering where the old man is. The narrator feels not the least bit anxious. As he puts it:
I smiled, --for what had I to fear? I bade the gentlemen welcome. The shriek, I said, was my own in a dream. The old man, I mentioned, was absent in the country. I took my visitors all over the house. I bade them search --search well. I led them, at length, to his chamber. I showed them his treasures, secure, undisturbed. 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.
The narrator believes he has won, and in fact, he seems as if he will get away with the crime. The police believe his story and do not suspect him of evil. However, because of his guilt, the narrator imagines he hears the heart of his dead victim beating, louder and louder. It gets so loud that he can't stand it anymore. He thinks the police hear the beating heart too and are playing with him by pretending they don't. Finally, the narrator snaps and confesses to the crime. His guilty conscience has gotten the better of him.