Mental illness is just another scapegoat for gun violence. There's no clear way to tell who is more prone to violent behavior and there's no real evidence that suggests mentally ill people commit gun violence at a greater rate than the general population. I personally know clinically mentally unstable people who wouldn't hurt a fly, clinical psychopaths and sociopaths who are no more prone to violence than anyone else, and otherwise normal people who I would have to keep an eye on if someone pissed them off the right way.