I think that it’s because the man had no reason to kill him other then his eye so he never acted on it because he was nervous but one night he just saw the opportunity and took it. So in short my answer is : He had no reason to other then the old man’s eye and he was nervous
Since the narrator is only vexed with the old man's eye, he doesn't kill the old man because during the first seven nights, the eye was closed. Since the eye is closed, he had no problem with the old man, nor a reason to kill him.
