In "The Tell-Tale Heat," what specifically is about the old man that troubles the narrator? What does the narrator do every night? why?

The narrator is specifically troubled by the old man's eye which is covered in a filmy substance.  

"The narrator claims that he loves the old man and has no motive for the murder other than growing dislike of a cloudy film over one of the old man’s eyes."

The narrator becomes convinced that the old man's cloudy eye is the evil eye capable of causing him harm. 

Every night, in the darkness, the narrator watches the old man.

"For a whole week, he has snuck into the man’s room every night, but the victim has been sound asleep with his eyes closed each time. The narrator cannot bring himself to kill the man without seeing his ‘‘Evil Eye.’’

"On the eighth night, however, the man springs up and cries ‘‘Who’s there?’’ In the dark room, the narrator waits silently for an hour. The man does not go back to sleep; instead, he gives out a slight groan, realizing that ‘‘Death’’ is approaching."   

The narrator is determined to get rid of the possible curse that the evil eye can put on him, so he decides to kill the old man.  That is why he stalks the old man in his room every night, until he is ready to kill him.