* Obsessive Behavior: The narrator fixates on the old man's "vulture eye" to the point of irrationality. His obsession consumes him, leading him to plan and execute the murder. This obsessive behavior is a hallmark of mental illness.
* Delusions of Grandeur: He claims his actions are motivated by a desire for "love" and a "passion" for the old man, yet his methods are cruel and sadistic. This suggests a distorted sense of reality and an inflated sense of self-importance.
* Paranoid Thoughts: The narrator believes the old man is aware of his presence and intends to harm him. This paranoia fuels his desire to silence the old man, leading to the murder.
* Hallucinations: He hears the beating of the old man's heart even after his death, suggesting auditory hallucinations.
While the story doesn't definitively diagnose the narrator's madness, these elements point to a psychological disturbance that likely stems from a combination of obsessive-compulsive tendencies, paranoid delusions, and potential hallucinations.
It's important to note that Poe intentionally leaves the narrator's motive ambiguous, leaving the reader to interpret the story's psychological complexities. The ambiguity contributes to the story's haunting and disturbing nature.