There's insanity, when a person truely doesn't know what they are doing is wrong, and there's the
Twinkie Defense version of insanity, which is more a legal trick than anything else.
In so many cases, insanity pleas seem to absolve a person from responsibility, especially 'temporary insanity' cases, or heat of the moment, that the person is normally a rational human being but something set them over the edge for that moment. That I don't buy, I have a temper, but I have never lost control of my temper to the point where I wasn't aware of what I was doing.
A person is either guilty or not guilty. Extenuating circumstances may have them be insane, but it doesn't make them any less guilty.