What prevents the narrator from killing the old man in the first seven nights in "The Tell-Tale Heart"?

In "The Tell-Tale Heart", the narrator claims that his primary motive for killing the old man is the old man's hideous "Evil Eye", which frightens and horrifies him. He is convinced that they eye itself is evil and determines that he must extinguish it. Because his issue is with the "Evil Eye" and not the old man himself, the narrator feels that the "Evil Eye" must be open in order for him to go through with the murder. On the first seven nights, the narrator enters the room silently and the old man is asleep so that he never has the opportunity to extinguish the eye. Finally, on the last night, the narrator makes a noise when opening his lantern which wakes the old man. The narrator shines his lantern upon the "Evil Eye" and finally has the opportunity to extinguish it by killing the old man.