I'm sure the psychologists did give Casey a mental health disorder diagnosis. What they didn't do is diagnose her as" insane." Which means if she is found to be guilty she, by law will be held accountable for her own behavior. She clearly has the ability to distinquish between right and wrong and thus is not insane, just mentally disturbed.
I think if she was to be diagnosed using the new Diagnostic and Statistical Manual of Mental Disorders (DSM-5) available May 2013 she would most likely be diagnosed as: Antisocial/Psychopatic Type. This diagnosis will include the following explanation:
Individuals who resemble this personality disorder type seek power over others and will manipulate, exploit, deceive, con, or otherwise take advantage, in order to inflict harm or to achieve their goals. An arrogant, self-centered, and entitled attitude is pervasive, along with callousness and little empathy for others needs or feelings. Rights, property, or safety of others is disregarded, with little or no remorse or guilt if others are harmed. Emotional expression is mostly limited to irritability, anger, and hostility; acknowledgement and articulation of other emotions, such as love or anxiety, are rare. There is little insight into motivations and an impaired ability to consider alternative interpretations of experience.
Temperamental aggression and a high threshold for pleasurable excitement are typically associated with this type, linked to reckless sensation-seeking behaviors, impulsivity without regard for consequences, and a sense of invulnerability. Unlawful or unethical behavior is often pursued, including substance abuse and physical violence. Aggressive or sadistic acts are common in pursuit of personal agendas, and sometimes pleasure or satisfaction is derived from humiliating, demeaning, dominating, or hurting others. Superficial charm and ingratiation may be employed to achieve certain ends, and there is disregard for conventional moral principles. General irresponsibility about work obligations or financial commitments is commonly present, as well as problems with authority figures.
I think this is an apt diagnosis for Casey. Just my humble opinion.