It was really strange for me to see the past in colors and the present in black & white, then I discovered how tension is immersed in the present to make it gray. No matter how your definition of "Fun" is, It's not the kind of "Fun" which amuse the "natural" audiences, it is a fun.. but that satisfying mental patients or murderers. Alicia Witt was absolutely great, she was invoked completely in the role, while William R. Moses played a nice a developed role that he showed some essential emotions towards his patients.
Rafal Zielinski dug a lot inside the psychological nature of born killer as it was a manner to express themselves in, via these two teenagers, we cannot trust what they said about themselves, or the torture and rape they might suffer with their families, because they didn't have the disconnection dissociation between realities and fictions, but we can feel how much they were in need for love, fun, happiness and personality.
Good metal music and editing, love the way of telling the event with flashback. It's fun to watch it, though the irony is it's a murder for fun!