I came across an incident in the Deseret News today of a man, reportedly with mental illness, taking and ax to his mother and slitting her throat.
http://www.deseretnews.com/article/865603316/Ax-wielding-Kearns-man-attacked-his-mother-police-say.html
Apparently they were having a family conflict.
Incidents like this give the impression that persons with mental illness are more prone to violence than others, however each person is an individual.
Use of drugs, especially if co-occurring with a mental illness can indeed lead to more violence, but it it does not follow that people who do drugs and have mental illness are always violent.
This article gives some insight.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1525086/
"Are the public at risk? Mental disorders are
neither necessary nor sufficient causes of violence. Major determinants of
violence continue to be socio-demographic and economic factors. Substance
abuse is a major determinant of violence and this is true whether it occurs
in the context of a concurrent mental illness or not."
It is important to for a mental health worker to access for violent risk; danger to self and others on a frequent basis. However it is just as important not to assume that because someone has mental illness that they will be violent.