Studies show about 25% of people will suffer a significant mental disorder during their lifetime.  It's part of the human condition, which is worsening in modern times with its increasing stresses.  Get use to it.  That said, most disturbed people do not turn violent, and the ones that do commit violence should be prosecuted and jailed, receiving therapy while incarcerated.  Mental illness should not be a "get out of jail free" pass.
http://www.who.int/whr/2001/media_centre/press_release/en/