In this eerie short story, the narrator tries to convince the reader that not only is he sane but is also justified in killing an old man, whom he claims to love, because of his eye. After carefully plotting out his murder and waiting for just the right moment to execute it, the narrator kills and then dismembers the old man, storing his body parts underneath the floorboards. At first he believes that he's totally gotten away with the murder, but when the police show up, he hears the beating of the old man's heart, which is really a manifestation of his own guilty conscience, and he ends up confessing everything.
One conclusion based on the outcome of the story is that it's nearly impossible to get away with evil deeds and that the truth will eventually come to light. The narrator isn't really hearing the beating of an old man's heart after he died; instead, he is becoming increasingly guilt-ridden over his crime, and that guilt manifests itself in a physical and tangible way.
Another conclusion is that desperate people will find ways to justify the behavior that allows them to fulfill their desires, no matter how irrational. In the beginning, the narrator seems to land on his reason for murder almost by accident:
He had never wronged me. He had never given me insult. For his gold I had no desire. I think it was his eye! yes, it was this!
Rational readers understand that a "pale blue eye, with a film over it" is surely no cause for murder, but somehow the narrator develops a belief in this nonsensical reason for taking this man's life, and his resolve is absolute. It is impossible to rationalize the plans of insane men.