Answer:
He couldn't stand the old man's vulture eye.
Explanation: The narrator is said to have been fond of the old man, but is driven mad by the mental issues he said has been caused by the "evil eye". He later kills the old man in a week's time, and when is investigated, keeps his cool until he hears the mental thump of the old man's heart. His own wicked desire to kill the old man brings him to his knees, leading to him being found out by the officers.
