The issue is that mental illness is generally tragic, not entertaining. When one is in the business of crafting entertainment, poetic license is to be expected.

Media perception of murderers amplifies the problem: you've heard of Lyle and Erik Menendez. You've heard of Scott Peterson. You've not heard of Ricky Abeyta, despite the fact that he killed twice as many people as the other three combined. Why? He didn't do it with any drama. He just went blood-rage and slaughtered a couple families. Only reason I know about him is it happened down the street from my grandparents' house.

We're not particularly concerned with violence perpetrated for a reason we understand - we're interested in violence perpetrated for a reason we have to puzzle out. Thus the fascination with serial killers over mass murderers. Shoot up a Hollwood Video? nobody remembers. Make lampshades out of people's skins? Now we're talking a string of cheap slasher films.

There's also an aspect of "ookyness" to mental illness; while schizophrenia is represented in 1% of the population of the United States, fully a third of all homeless people suffer from it. And while it's true that schizophrenics are more likely to suffer from violence than they are to commit it, they're also a lot more likely to commit it than sane individuals. There's disagreement about whether this is fundamental to the illness or an associated symptom due to an increase in substance abuse (self-medication) and an attendant participation in crime. We shun the mentally ill; florid portrayals of the mentally ill as "other" is probably more an effect than a cause.