i can see being RESPONSIBLE because of mental illness... but not NOT RESPONSIBLE.
It’s a form of insanity defense. Incapable of telling right from wrong, so can’t be a criminal. One of the elements of most crimes is “knew (or should have known) it was wrong and did it anyway.”
Your honor, I can’t be a criminal. I’m a Democrat!
Years ago there was an attempt to get a lawful new verdict in such cases. “Guilty but insane.”