Glenn Close has a great article for HuffPo on the stigma of mental illness.
Even as the medicine and therapy for mental health disorders have made remarkable progress, the ancient social stigma of psychological illness remains largely intact. Families are loath to talk about it and, in movies and the media, stereotypes about the mentally ill still reign.
Whether it is Norman Bates in Psycho, Jack Torrance in The Shining, or Kathy Bates' portrayal of Annie Wilkes in Misery, scriptwriters invariably tell us that the mentally ill are dangerous threats who must be contained, if not destroyed. It makes for thrilling entertainment.
There are some notable exceptions, of course -- Dustin Hoffman in Rainman, or Russell Crowe's portrayal of John Nash in A Beautiful Mind. But more often than not, the movie or TV version of someone suffering from a mental disorder is a sociopath who must be stopped.