Just because you have a mental or emotional challenge does not mean that you cannot control your actions. Prime example, a pedophile is attracted to children. That is an emotion (very strange and sick one to most of us, as it should be) and part of their thought process. It does not cause them to sneak, groom, plan out, seduce, and hide their crimes. They know this is wrong. They know they are hurting the child(but they usually delude themselves into thinking they love the child, and the child wants it). They hide the crime (thus showing they know right from wrong).
There are quite a few men in the US in therapy for having thoughts of Pedophilia that have never acted on it. Just because you are abnormal or seeing the world in a way most people do not, does not mean you do not know right from wrong. The ones who have commited crimes like serial killing, and the like have no empathy for other people. They still know it is wrong, they just don't care unless it will harm their quality of life.
I honestly believe after watching Casey she has no empathy(only a good guess on my part, but I would bet on it). If they want to get through to her, then negotitate using her state of existance as a barganing tool. No death penalty, better food, softer bed, more family visits, less family visits, prison near home, whatever makes her happy(within reason here). Otherwise she will never open up, because it does not make her life easier.