Complex sentence: The book makes the case that most serial killers are not guilty of their crimes by reason of insanity, and that they are incapable of understanding the evil of their actions, despite being, in many cases, highly functioning individuals in society.
Split into simpler sentences: The book makes the case that most serial killers are not guilty of their crimes by reason of insanity. And that they are incapable of understanding the evilness of their actions, despite being, in many cases, highly functioning individuals in society.