Does mental illness remove all responsibility for a crime?

It's interesting to me that so many of those commenting on the Arizona shooting of Rep. Gabrielle Giffords and 19 others seem confident in the diagnosis that Jared Loughner is mentally ill.

It's an easy "diagnosis" to make if you are not a mental health professional, and even easier if you have not examined the patient.

The difficult societal question here is: How do we separate mental illness from individual responsibility? Is Loughner mentally ill? If so, what it his diagnosis? Is that a diagnosis that often, or rarely, leads to violence?

Further, does Loughner--even if he is mentally ill--bear any responsibility for his action? People with mental illness are impaired, but that doesn't meant that they bear no responsibility for their actions. Their illnesses can affect their behavior, but rarely do such illnesses remove from them all ability to choose what they say or do.

Was the Arizona shooting the product of a man so mentally impaired that his illness drove his car, directed his feet to the gun counter, put down his payment, wrote his warning notes, and fired that gun that it had put in his hand? We will never know for sure, but it might be the case that even if he had a propensity to violence, he could have chosen not to act on it. That would put at least some of the responsibility on him, not on his illness.

Easy answers and armchair diagnoses are likely to be wrong. If we want to prevent this in the future, we need to assess very carefully what happened in Arizona.