Drug and alcohol.dependency are the things you need to worry about, not mental illness.
http://research.bmh.manchester.ac.uk/cmhs/research/centrefor...
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> xx. In all four UK countries, most patients convicted of homicide also have a history of alcohol or drug misuse, between 88% in England and 100% in N Ireland. In other words it is unusual for mental health patients to commit homicide unless there is a coexisting problem of substance misuse.