One can make an argument that all criminals are insane. One standard for insanity is based on whether the person would commit the crime knowing he would be caught, as in, even though there was a policeman standing next to him. An important rule of criminal law is that while ignorance of law is not an excuse, ignorance of fact is an excuse.
Since there is reason to believe that most criminals are unaware that Ha-Shem rules the world, and is constantly observing their actions, and punished wrong-doers, most criminals should be considered insane since they lack knowledge of the reality that Ha-Shem is present and judges their action.
Frankly, hanging makes a lot more sense.