"a single dim ray, like the thread of the spider, shot from out the crevice and full upon the vulture eye. It was open—wide, wide open—and I grew furious as I gazed upon it."
"And now a new anxiety seized me—the beating of his heart would be heard by a neighbor! The old man’s hour had come!"
It is clear that the protagonist is suffering from some mental illness. He did not commit murder because he hated his victim, he just got fixated on the old man's eye. He deemed it evil and wanted to get rid of it staring at him. As he said "I loved the old man. He had never wronged me."He called it mostly "Vulture eye" and "Evil eye" while the eye was highly likely normal to any other person. He repeatedly saw things because of his paranoia and focused way too much on them. First it was the evil eye and then it was the heartbeat. After he killed the old man in the middle of the night, police officers visited his house because they were alerted by neighbors who had heard the victim scream. He had hidden the victim's body and was confident that the policemen would find nothing suspicious. He told them that the scream was his own because of a nightmare he had seen and invited them to seat with him in the old man's room and chat. Everything was normal at first, however after some time he started hearing a loud heart beating and got an anxiety crisis thinking it was the old man's heart. The beating got louder and louder while the policemen were still sitting there and talking. Finally he lost his temper, thinking that the policemen heard the beating too and just mocked him while already knowing that he was the murderer and admitted the crime. “Villains!” I shrieked, “dissemble no more! I admit the deed!—tear up the planks!—here, here!—it is the beating of his hideous heart!” . However the reader knows that it is impossible for the heart of the dead man to beat so it is considerably clear that the heart beating was the protagonist's because of having an anxiety attack. The story ends by him admitting a crime to the policemen which would probably not be discovered in any other way. It is an interesting story that portrays the mentality of a psychotic person and shows in what ways the brain can malfunction when one has lost their sanity. Such characters are often unpredictable and consequently very interesting when used in stories.