What do you really think caused the narrator to confess?

I think it is his guilt that causes the narrator to confess. In the end, he has gotten away with the murder. The police are fooled by his calm and confident demeanor, and they "chatted pleasantly, and smiled." Suddenly, though, the narrator "grew very pale" and he begins to talk more quickly. He feels that he is gasping for breath and gesticulating violently; he says, "I foamed—I raved—I swore!" In reality, he does none of these things, and they are all taking place within his imagination as the men just keep sitting there. The narrator hears a sound he has heard before: "a low, dull, quick sound—much such a sound as a watch makes when enveloped in cotton." He thinks this sound is the old man's heartbeat still beating underneath the floorboards, but we know that this is not possible. The heartbeat the narrator hears must be his own, and it would be racing, thumping loudly if he were feeling guilty for the murder he has committed. Therefore, I believe it is the narrator's guilt that compels him to confess because a) he was just about to get away with the murder, and b) his quickened heartbeat could indicate a guilty conscience.