"I smiled- for what had I to fear" Is what the narrator said to himself when there police at his door. The narrator had just killed the old man the night before so he has many things to fear like what if they find his body?
The narrator watched the old man over and over again for seven nights at midnight. This is again and again because the narrator watched him over and over again for a whole week.
The narrator thinks he's hearing the old man's and he starts going crazy. The narrator goes crazy and tells the cops that he killed the old man.