Peeping Tom (1960)

Loner Mark Lewis (Carl Boehm) works at a film studio during the day and, at night, takes racy photographs of women. Also he's making a documentary on fear, which involves recording the reactions of victims as he murders them. He befriends Helen (Anna Massey), the daughter of the family living in the apartment below his, and he tells her vaguely about the movie he is making. She sneaks into Mark's apartment to watch it and is horrified by what she sees -- especially when Mark catches her.