The narrator in "The Tell-Tale Heart" is driven to murder, yet he isn't quite sure what drives him to do it:
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!
Seemingly needing to convince himself that the man deserves to die, the narrator ultimately blames his plans for murder on an eye that is "pale blue" and covered with "a film."
He also finds that he cannot murder the old man while he sleeps. With the pale blue eye covered during the man's slumber, his motive for murder is extinguished. After all, he ruminates, the old man himself is not the issue—the "evil eye" is the source of conflict.
For seven nights, the narrator sneaks to the old man's bedroom door. He brags about how stealthily he approaches the room, taking an entire hour to place his entire head into a small opening so that he can observe the man's appearance. Using just a faint light from the lantern, the narrator ascertains whether the eye is open or closed, and for seven consecutive nights the "vulture eye" cannot be seen as the man soundly sleeps.
This changes on the eighth night. The narrator performs his ritual of observation and finds that the man has awakened, calling out, "Who's there?" as the narrator waits in the darkness, perfectly still and silent. When he finally shines the light of the lantern toward the old man, he finds that the eye is wide open, which makes the narrator furious as he looks upon it. This provides the motive he's been waiting for, and he kills the man in order to rid himself of the eye which vexes him.
The narrator's incredibly stealthy plans to murder a man because of the appearance of an eye point to his madness. It is also worth noting that while he feels he cannot kill the old man while he sleeps, he takes extreme precautions in order to avoid waking him for seven nights in a row. It would seem that if he needed the man's eye to be open in order to kill him, he would simply enter his room noisily. This points to an inner subconscious conflict within the narrator, demonstrating that his conscience struggles with his murderous plans.