Arguably, the main idea of "The Tell-Tale Heart" is that when you commit a crime, like murder, you cannot escape your guilty conscience, no matter what the circumstances or your state of mind. Take the narrator's motivations, for example. He believes wholeheartedly that the old man he lives with has an "evil eye." This eye bothers him so much that it makes his blood run cold. The idea of murder is so strong in the narrator's mind that it "haunted" him day and night, and, more importantly, he believes that it is the only way of escaping this evil eye.
In terms of the murder itself, the narrator goes to great lengths to dispose of the old man's body and to evade detection by the authorities. Even when the police visit the house, the narrator has no problem convincing them of his innocence.
The sound of the beating heart, however, is so powerful that the narrator cannot ignore it. This sound functions as a symbol of his guilty conscience. Despite his dedication to not being caught by the police, the narrator is powerless to ignore this sound. He is literally deafened by the sound of his internal sense of guilt.
Through the experience of the narrator, then, Poe argues that guilt is not only a strong emotion but one that overpowers all others.