I think you have that inverted: Mentally ill people are not necessarily evil and do not necessarily do evil. But evil causes much mental illness: the evil of an amoral, G-dless society, opening up pathways to drugs, sexual addictions and perversions and the complete devaluation of human life as sacred and precious. These are the ingredients of minds coming unhinged.
I also believe that these VERY VIOLENT video games these kids are spending hours and hours on are making them completely numb to the value of life in the real world!!