One of the oldest jokes told about the insanity defense is that the insane don't believe that there is anything wrong. It is also true that there are a lot of people close to the edge and usually they know as much which is why they haven't crossed over. As far as being a defense, unless it can be proved that the person involved was out of their wits before, during and after a crime was committed that was a direct result of the symptoms of that form of insanity present in the perpetrator, usually there is only a suspended sentence involved and the first part is spent in a mental institution until the person is deemed rational enough to server their sentence. It is pretty hard to convince an attorney, judge, and jury that actual crimes, especially capital crimes, are committable by anyone without a conscience. Even in such cases as the insanity defense is provable, the sentence usually includes a life time probation of sorts.