HereSince1628 put together a really good post on crime and the mentally ill, but he has vacated the group and taken all of his posts with him. I'll see about trying to re-create it here. The conclusion that he came to was that mentally ill people are far more likely to be victims of crimes rather than perpetrators.
Here's a fact sheet about mental illness and homicide in New Zealand, Australia, and the UK:
"It is a myth that offenders with a mental illness are more likely to kill a stranger than offenders without a mental illness."
http://www.forlagetamalie.dk/pdf/factsheet2.pdf
Lots of good info on this site about mental illness, stigmatization, and crime, including:
"According to a study by Northwestern University, nearly 3 million people in the US, who are severely mentally ill become victims; one-fourth are victims of violent crimes, 11 times higher than the general population. People with mental illness are 8 times more likely to be robbed, 15 times more likely to be assaulted and 23 times more likely to be raped. Theft of property from persons, rare in the general population (0.2 %) happens to 21% of the mentally ill. A minor theft increases their anxiety and worsens psychiatric symptoms."
and
"Research shows people in treatment for a mental illness are no more violent or dangerous than the the general population. In Sensationalizing Murder and Mental Health John M. Grohol, Psy.D. states, " ... there is virtually no correlation between increased violence risk and mental illness (except in the case of substance abusers)." (Psych Central)"
http://karisable.com/crmh.htm#stigma
Interestingly, in my search I ran across a site proclaiming "Nearly 1 in 5 who commit violent crimes are mentally ill." I'm sure you can see the slant. It turns out that about 26% of the population suffers from some form of mental illness. So, proportionately, less mentally ill people commit violent crimes than sane people.
Another study on the victimization of mentally ill people. The contrast is stark.
RESULTS:
More than one quarter of persons with SMI had been victims of a violent crime in the past year, a rate more than 11 times higher than the general population rates even after controlling for demographic differences between the 2 samples (P<.001). The annual incidence of violent crime in the SMI sample (168.2 incidents per 1000 persons) is more than 4 times higher than the general population rates (39.9 incidents per 1000 persons) (P<.001). Depending on the type of violent crime (rape/sexual assault, robbery, assault, and their subcategories), prevalence was 6 to 23 times greater among persons with SMI than among the general population.
http://www.ncbi.nlm.nih.gov/pubmed/16061769
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