Be specific. "Mental illness" is not a good catch-all term for a variety of different conditions. Depression tends to cause violence towards the self - suicide and self-harm - but not violence. What specific mental conditions do you have in mind and how have they been diagnosed in the people under discussion?

No, it doesn't.

Violence is common. Mental illness is common. Some people happen to have both, but there's few of them where the violence is caused by the mental illness.

When looking at things that predict violence we see that substance misuse is a better predictor. Previous exposure to violence (as a perpetrator or victim or witness) is a better predictor. Either of these combined with untreated SMI where the patient is not in contact with services is a stronger predictor. But SMI itself is only a weak predictor of violence.

You can read the NCISH (National Confidential Inquiry into Suicide and Homicide by Mental Health Patients) data for a research based look at the data.

https://documents.manchester.ac.uk/display.aspx?DocID=38469

EG, from the summary on page 6

> most patients had a history of alcohol or drug misuse; homicide in the absence of comorbid substance misuse is unusual

> around half of patients were not receiving care as intended, either through loss of contact or non-adherence with drug treatment

> patients are also at high risk of being victims of homicide

And for a more researchy source: https://jamanetwork.com/journals/jama/fullarticle/183929

> In patients with schizophrenia, 1054 (13.2%) had at least 1 violent offense compared with 4276 (5.3%) of general population controls (adjusted odds ratio [OR], 2.0; 95% confidence interval [CI], 1.8-2.2). The risk was mostly confined to patients with substance abuse comorbidity (of whom 27.6% committed an offense), yielding an increased risk of violent crime among such patients (adjusted OR, 4.4; 95% CI, 3.9-5.0), whereas the risk increase was small in schizophrenia patients without substance abuse comorbidity (8.5% of whom had at least 1 violent offense; adjusted OR, 1.2; 95% CI, 1.1-1.4; P<.001 for interaction). The risk increase among those with substance abuse comorbidity was significantly less pronounced when unaffected siblings were used as controls (28.3% of those with schizophrenia had a violent offense compared with 17.9% of their unaffected siblings; adjusted OR, 1.8; 95% CI, 1.4-2.4; P<.001 for interaction), suggesting significant familial (genetic or early environmental) confounding of the association between schizophrenia and violence.