Maniac

This film is pretty similar to Peeping Tom or Psycho, in that it features a serial killer who is haunted by the memory of one of his parents.  In this case, it is his mother, and he removes the scalps of the women he kills to put on mannequins in his apartment, all while talking to himself from his mother's point of view, or talking to his imaginary mother.  It's a pretty disturbing film with some gruesome deaths, and the low budget style adds a kind of eerie realism to the story and captures the early 80s New York City nighttime vibe.  The killer in the film is actually a pretty unattractive guy, with a mustache, greasy hair, and a big beer belly, which is different from the more normal looking killers in Psycho and Peeping Tom.  He's also always moaning and panting while he stalks the women, which adds another layer of creepiness.  Apparently, the director of this movie directed a bunch of hardcore porn films before this, and I guess that shows in the overall style and content.  Joe Spinell, a character actor who is also in The Godfather Part II and some of the Rocky movies, does a pretty good job as the maniac.  Overall, I would recommend this if you want a pretty creepy, gory, serial killer-type movie.