*spoilers* who believes that the Killer is....
Jess played by Olivia Hussey.
Before people jump at me, think about it. Psychosis can be triggered and murderous fantasies by obscene phone calls that can set Jess to commit real life horrors, it gives her an opportunity to commit hedonistic crimes.
She can make up her own phone calls, they were different from the lewd first call, later on they morph into more personal issues and consistency had changed in play acting voices like puppetry. The viewer can't see she is doing them, it is a guessing game.
Jess is cold and be cruel and plays on others emotions, and surprisingly lacks warmth.
Could fake Pregnancy all along.
Framed Peter.
Not afraid to attack or be as vulnerable as people think
The last one standing out of the bevy group.
Ant the end the Killer mocked about being missed out and avoiding going to the hospital in case she is bluffing. She could say she lost her Baby because of stress and trauma.
What do you think?