If a criminal pleads insanity he will probably avoid the wrath of the public: he frees them from their guilty conscience. The people can then say: he is not our problem, we are not the cause, he is simply insane. With such a designation they distance themselves from him, treat him as an independent existence, and therefore avoid responsibility for him. To accept the responsibility for such ones would mean having to see the madness in themselves.
Criminals are in fact quite normal, because crimes are committed with the goal of happiness in mind, which is, after all, a normal human motivation - isn't it?
We sane people of the world live in a country called "sane" and are therefore in-sane.