Typical Scenario:
So things aren't looking so great: The cops caught you stabbing a priest, which you have found out is illegal in your city. Fortunately, your lawyer is Richard Gere, and the two of you combine to convince the jury that you are insane, and that the crime was carried out by one of your multiple personalities.
The man who defends himself has a fool for a lawyer. The man who hires Richard Gere has a sentient nose for a lawyer.
You get off, innocent by reason of insanity! Congratulations, a couple of months at the mental hospital and you'll be back on the streets!
Why it's BS:
Obviously if the legal system had this kind of Get Out of Jail Free loophole, the prisons would be empty. In reality, the Insanity Defense is attempted in less than one percent of all legal cases, which essentially means that more people have tried to pin their crimes on aliens or their evil twin rather than their own basket case, shoelace-eating lunacy.
"No no, it was my brother, Larry Busey!"
Of that tiny fraction where the lawyer was even willing to try it, the defense is successful less than 25 percent of the time. Three states in the US don't even allow insanity as a defense.
Then, in that tiny, tiny fraction of cases where the guy "got off" because he convinced the court he was insane, he doesn't get to just go home. You get sent to a mental institution where you don't have a set sentence at all--they keep you as long as they see fit, which may be forever. You're there until "deemed safe to return to society", which according to the American Psychiatric Association is usually twice as long as the jail sentence would have been.
Hope you like white gowns and staph infections!
This has always been one of those situations where people simply want to believe the system is worse than it is. We can't escape the idea that the courts are too soft on the bad guys and that guilty people are running free left and right. Besides, what is insanity, anyway? Can't you say that anyone who willingly kills another human being is "insane"? Won't giving these guys a free pass based on a little craziness bring the whole system down?
Yes, it would. Which is why the courts don't do it.