Mentally challenged zombie

What exactly is the entertainment industry trying to tell us about people with mental issues? The first thing that comes to mind is, obviously, Hannibal Lecter. His depiction is not one of a human being, but one of a monster. That mask is actually supposed to prevent him from biting people, for he is actually a cannibal. It's basically a muzzle. They're just treating him like a rabid dog. And him helping the police is not compared to Sherlock Holmes solving cases as much as it is seen as the police desperately using a dangerous and unstable weapon.
Even worse than Lecter must be the sociopath that the other sociopaths and psychiatrists run away from out of pure fear - the Joker. With his dread pale face, crimson red lips and poisonous green hair, the Joker doesn't even look like a human being. He could be thought of as death - or alternatively, dead. He's supposed to look like a clown, isn't he? And a popular explanation for coulrophobia is that clowns look like corpses or zombies. Remember that in his (usual) origin story, he fell into an acid vat. As everyone knows, acid is hazardous for life. The Joker should thus be dead by now. In fact, he is dead, or so he said in the 1989 movie.

Jack Napier is dead. I am the Joker.

~the Joker

The Joker is not as much evil as he is outside nature, something science can't explain. That's actually quite a metaphor how general populace sees mental problems - or rather, madness. The word 'mad' was originally used of them before it was understood that it was a phenomenon. When the word was replaced with 'mentally challenged' people simply thought that it's something doctors and therapists could comprehend, not regular people. They could still avoid the issue of talking about it, since it was a taboo. That continued until recently 'mentally challenged' became a taboo too and psychiatrists started to use the more precise terms such as 'bipolar disorder' and 'manic-depressive' and so on. Now it was something to understand, to be explained. Yet this didn't prevent Hollywood and the such from continuing to use the character archetype.