Fan theory [spoilers]

From IMDb trivia;

There is a fan theory that the hospitalized daughter of the movie's killer (Peter Bunch) is actually the film's true antagonist and that she is using psychic powers to control her father and make him murder and torture innocent women out of jealousy. This is backed up by two things; near the end of the movie, while Peter Bunch is about to kill a women, he yells "You women are all the same! Young, beautiful and healthy, while I have to rot to death in bed at a stinking hospital!", which is an unusual thing to say, considering it is actually his daughter who is dying in hospital, not the father himself. Lucio Fulci also previously directed The Black Cat, which came out a year before The New York Ripper, and tells the story about a man with psychic powers to make a black cat kill a bunch of people out of revenge.