In "The Tell-Tale Heart," what prevents the narrator from killing the old man during one of the first seven nights?

The narrator says that his reason for killing the old man is that he does not like the old man's "pale blue eye, with a film over it" - he calls it a "vulture eye" and an "Evil Eye". For the first seven nights, the narrator goes through his elaborate measures to sneak into the old man's room at midnight without the old man noticing: he carefully unlatches the door and opens it, puts in...

The narrator says that his reason for killing the old man is that he does not like the old man's "pale blue eye, with a film over it" - he calls it a "vulture eye" and an "Evil Eye". For the first seven nights, the narrator goes through his elaborate measures to sneak into the old man's room at midnight without the old man noticing: he carefully unlatches the door and opens it, puts in a lantern and then pokes in his head, and then undoes the lantern so that a ray of light falls upon the old man. There was only one reason why the narrator did not kill the old man those first seven nights: the old man's eye was closed, so the narrator could not see it, which he claimed made it "impossible to do the work" because he had no problem with the old man, but with his blue eye. On the eighth night, the narrator accidentally awakened the old man, and after the old man had fallen back asleep, his blue eye was still open, and it infuriated the narrator enough to make him kill the old man.