What was The narrator in the tell tale heart obsessed with?

In Edgar Allan Poe's "The Tell-Tale Heart," the narrator is obsessed with the old man's eye, which he describes as a "vulture eye" that causes him intense discomfort and paranoia. This fixation drives him to commit murder, believing that by eliminating the old man, he can rid himself of the eye's haunting presence. His obsession reveals his deteriorating mental state and ultimately leads to his downfall as he is consumed by guilt and the sound of the old man's heart.