As I read the book, I talk about my reaction when I realized who the killer was there. It's true that in the show the clues may not as clear as in the book (I remember they always talked about Adora's floor and I didn't realized that the girls' teeth have a connection with it until the last page xD), but I think it was brilliant when they maked us believed it was Adora, and just in the very last moment you realized it wasn't her, but Amma! She always was a two-faced girl but seemed incapable of doing such brutal things, so the shock was huge! :D