Henrik,
With all respect, mental illness and being mentally disturbed still come under the same umbrella. Which ever excuse is made for an act of crime against a non-mentally ill or non-mentally disturbed amounts to the same net result, surely?
I agree with the suggestion that the system is 'vague' in dealing with mental illness. I believe that psychiatry is not an exact science and is, in law, shackled by that same law that serves us all no matter in which Country.
I can't agree with your suggestion that it would be barbaric to hold the perpetrator guilty of a crime, no matter what thier personal circumstance was. The crime has been committed and the victim remains the victim.
However, is good to debate!

Peter M.