Marijuana is beneficial for a number of conditions; I'm not aware of any research that shows that violent psychopathology is one of them.
Yeah, well marijuana maybe beneficial for some conditions, but there are medicinal versions of the drug that can be used without smoking it.
The case in Bellingham, the guy was psychotic and under medical supervision, but decided that it was better to self medicate with marijuana, as the free drug crowd had told him that marijuana would cure what ailed him and make everything ok. He went nuts, and tried to kill his mother and sister. The father tried to stop him and got killed, instead. Marijuana is not a cure for psychosis, violent or otherwise. This man had not been violent before.