It first appears when he shines the lantern's light on the old man's eye. It is the lantern shining on the eye that spurs him to kill, in contrast to the previous nights where the eye had remained closed. The beating heart is the narrator's response to the desire to kill -- a reminder that the old man is a human being. The narrator misinterprets the beating heart and kills the old man, but the heart does not stop beating. The old man's humanity has not been extinguished with his life. In his subconscious, the narrator realizes this, which is why the heart torments him. Cognizant of the old man's humanity, the narrator thus retains a fragment of his own. The heart's beating ultimately compels his confession. In this way, his conscience speaks to him. The sane part of the narrator feels guilt over the act, and the confession is the narrator's conscience accepting culpability for the murder.

Poe's narrator derives guilt from his underlying humanity. The...

The narrator's insanity has compelled the murder, but his conscience cannot accept that he is a murdering madman, so he not only confesses to the crime but he creates what he believes to be a rational justification for an irrational act. Conscience is portrayed by Poe in this story as the underlying humanity in all of us. He uses the insanity of the narrator to illustrate how a person's conscience can be isolated from their actions. This isolation Poe uses to explore the darker side of humanity, that the degree to which conscience is present or lacking in an individual can determine the course of their actions. We can easily see that a man of greater conscience would not have committed the murder, and a man of less conscience (more insane) would not have confessed. That the narrator has a shred of remaining humanity is ultimately his undoing as criminal.