(1) If a person kills or is a party to the killing of another person, he or she shall not be convicted of murder if he or she was suffering from such mental disorder (whether arising from a condition of arrested or retarded development of mind or any inherent causes or induced by disease or injury) as substantially impaired his or her mental responsibility for his or her acts in doing or being a party to the killing.
(2) On a charge of murder, it is for the defence to prove that the person charged is by virtue of this section not liable to be convicted of murder.
(3) A person who, but for this section would be liable to be convicted of murder, is liable to be convicted of manslaughter.
(4) The fact that one party to a killing is by virtue of this section not liable to be convicted of murder shall not affect the question whether the killing amounted to murder in the case of any other party to the killing.