When I was practicing law in California, attorney's would often joke about the "Twinkie Defense." The "Twinkie Defense" refers to a murder case out of the Bay Area where a disgruntled politician killed other politicians, and then beat murder charges in trial because eating too many Twinkies made him temporarily insane. Needless to say that after that case, California changed its laws regarding mental defenses.
I personally did a jury trial a few years ago that focused on the mental competency of a man charged with murder. The defendant charged with murder was found competent, despite some crazy things that he said and did. The key to the case was not so much what he did when speaking to psychologists and experts, but what he did the rest of the time, playing cards with fellow inmates, and how he behaved and talked when he didn't think he was being watched or evaluated.
When someone fakes a mental illness to avoid the consequences of bad choices, it is call malingering. Here in Utah, we have a trial underway involving the kidnapping and rape of a young woman named Elizabeth Smart by a man named Brian David Mitchell. The testimony in the trial to date has shown that, despite some strange behavior by Mr. Mitchell, he knew what he was doing. Today's testimony revealed that he used some of his unorthodox beliefs to manipulate his wife in assisting him.
Is Mr. Mitchell crazy? Probably, but not in a way that reduces his responsibility for what he has done. There is a difference between acting different from the social norm, and being so mentally ill that you are not responsible for your choices and your behavior. The criminal justice system is full of people who do crazy things. The main question to be answered when evaluating a person's competence to stand trial and be held responsible for their conduct is not so much whether their conduct was crazy, but whether they knew what they were doing when they did it, crazy or not.