Joseph, that could be (not all mentally ill people actually commit crimes) but as I previously posted society would still have to be protected from him. A vicious animal is also not responsible for its actions but we do not let it roam free.
RY,
1. A defense will not do anything unless the court accepts it.
2. Who says what they would prefer? In a mental hospital they would probably not be in danger from other patients.
TLIK, you are correct. For example, David “Son of Sam” Berkowitz was diagnosed as paranoid schizophrenic. yet was found competent to stand trial, was sentenced to prison and sent to Attica (@RY).