The narrator himself gives the reason why he only killed the old man on the eighth night. He had been visiting his room at midnight for seven nights in a row expecting to see the old man's wasted eye open. The eye, "the eye of a vulture -- a pale blue eye with a film over it," had become an obsession with the narrator for, from the first time he had seen it, it made his blood turn cold. The eye horrified him and he decided to kill the old man and thus rid himself of having to see the horrible eye ever again, as he states,
I made up my mind to take the life of the old man, and thus rid myself of the eye forever.
During each of the seven visits, when the narrator shone a light upon the eye, he found it closed and thus relented from completing his insidious task. He obviously wanted the eye to be open before he proceeded. In his crazed state, the narrator clearly believed that the eye being open would be a signal for him to do his evil.
On the eighth night of the storyteller's surreptitious visit, he felt invigorated and proud of his intelligence and judgment. When he later shone the light on the sleeping man's eye, he found that it was open -- the sign that he had been waiting for for so long. He hesitated for a minute but then, disturbed by the sound of the old man's beating heart growing louder, went into a frenzy, jumped into the room, dragged the old man from his bed and pulled his heavy bed over him. The old man, clearly too frail and weak to push off the heavy object, suffocated and died.
The number eight in the story also has symbolic significance: in Christian culture, it is seen as the number for regeneration and new beginnings. It is evident that the narrator believed being rid of the old man and, therefore, his terrible eye, would mean a new beginning for him -- a life without the horror of being exposed to the old man's evil eye.
In certain cultures, the number eight is extremely significant. In the Jewish culture, for instance, the concept of man having the ability to transcend his nature is represented by the number eight. When the High Priest officiated, he wore eight garments. And, from their eighth day onward, animals could be offered as sacrifices. In Edgar Allan Poe's story, "The Tell-Tale Heart," there seems much importance given to the eighth day by the narrator. It is the day of completion. Poe's narrator writes,
Never before that night had I felt the extent of my own powers--of my sagacity. I could scarcely contain my feelings of triumph.
Not only does the narrator sense the significance of the eighth day, but the old man feels the power of this night as well. He knows that the narrtor is going to kill him.
It is something about the old man's "cloudy, pale blue eye" which incites madness in the narrator. He says that when the old man looks at him, his "blood turns cold". The narrator wants to see the "Evil Eye" once more before he kills the old man, but each night for the first week the old man is asleep when he sneaks into his room. Finally, on the eighth night, the old man is awake. The narrator shines a light on his eye and kills him.