Protect the public.

As the government claims that murders by the mentally ill are not increasing, we must assume then that the figure has long been quite a high one, although just how much I feel that such stats may be trusted is quite another matter. Perhaps if we were to use the definition of 'criminally insane' with regard to such offences, greater clarity would be achieved as simply being classed as 'mentally ill' may not be strictly relevant.
Killings - I am reluctant to use the word 'murders' - are committed most frequently by those known to be dangerous and judged incorrectly by professionals to be 'safe' and by those who fail to take essential medication.
[In the case of psychopathy, the case for prison as opposed to the alternatives is quite a strong one.]
A great proportion of this problem has its roots in the ill-conceived 'care in the community' programme which has cost many lives since its inception.
I think the public has the right to demand that once a major act of violence has been committed by somebody who is mentally ill, then they will never again be allowed their freedom. The problem at present is that a crime is seemingly diminished by a conclusion that there is any mental illness present. A gradation of the level - and potential dangers - of every mentally ill offender is probably essential.
In retributive terms, in the best scenario, sentencing needs to be entirely commensurate with the degree of free will involved. The overriding factor should be protection of the community, however. The rights of the public must always take precedence over the rights of those who commit crime.
No advantage should ever accrue from an ordinary criminal's legal team claiming that 'mental illness is present' - but achieving this in the face of the enormous amounts of dishonesty by solicitors and barristers who go beyond the mere calls of duty when defending clients, will make this extremely difficult and always sensitive.
I refer in particular to offences of GBH, manslaughter, arson and rape.We are not always necessarily referring to prison under such circumstances but rather, secure facilities.
The safety of the public must be paramount.
Of course, reverting to a policy of having specialist accommodation for all of the mentally ill who need or desire such care might help the situation enormously.

LINKS: http://www.timesonline.co.uk/tol/news/uk/article658445.ece
http://www.uea.ac.uk/~wp276/homicide.htm
http://www.inthenews.co.uk/news/news/health/homicides-by-mentally-ill-largely-avoidable-$460063.htm
http://www.sagb.co.uk/06.htm
http://www.dailymail.co.uk/pages/live/articles/news/news.html?in_article_id=558166&in_page_id=1770