The narrator cannot kill the old man for the first seven days because his eye is closed, and he only wants to kill him because of the Evil Eye.
The narrator peaks his head in the old man’s room very, very carefully so the old man won’t see. He does not want to kill the old man, but he has to because of his evil eye.
And this I did for seven long nights—every night just at midnight—but I found the eye always closed; and so it was impossible to do the work; for it was not the old man who vexed me, but his Evil Eye.
Of course, the narrator is crazy. He keeps telling us he isn’t, but he clearly is. Anyone who wants to kill a perfectly innocent old man just because he thinks his eye is evil has some mental instability.