It's hard for you to imagine because you don't have a split personality. You're taken on this journey through one person's eyes and then find out they're actually two people. You don't like this because this changes your whole outlook on the movie. Now you must go back and dissect every single part of the movie, some people don't want to do that. The killer didn't even know she had two personalities, neither did you. So then, you're seeing it from both perspectives but then again you're seeing that one person as two separate people, kinda trippy.