Why did the narrator kill the old man on the eighth night from the story The Tell-Tale Heart?

The old man's eyes were closed 7 days before. The narrator wanted the old man's eyes to be opened when he killed him. The old man's eyes were opened on the 8th day when he was doing a nightly ritual.

"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."