Batman’s home city of Gotham canonically contains a high-security prison. Many dangerous criminals have been incarcerated in Blackgate Penitentiary thanks to Batman’s crime-fighting, and various storylines (most recently the film The Dark Knight Rises) have involved those inmates breaking out to wreak havoc on the city. Another Gotham institution, however, features in more breakout storylines than Blackgate, and is much more well-known among casual fans of the Batman franchise: Arkham Asylum, which is technically a mental hospital but essentially functions as a prison for the criminally insane. The fact that a disproportionate number of Batman villains are imprisoned in Arkham – or were, before escaping – strikes me as very significant.
I feel that the history of depictions of mental illnesses and developmental disorders goes a long way toward explaining this. A common and very old trope in fiction is the madman or madwoman, a character whose thought process is deliberately written to be incomprehensible to the reader and to other characters. This type of figure is almost always presented as frightening and dangerous due to his or her apparently irrational behavior, and is often also depicted as animal-like or otherwise inhuman, which can further repulse the reader. An example from the literary canon of a character with these traits is Bertha Mason of Jane Eyre, described as savage and bestial; in Batman canon, the sci-fi/fantasy aspect of the superhero genre allows for villains like Killer Croc to exhibit the same traits more literally, making the mad(wo)man character even more of a demonic figure.
So the mentally ill character as object of fear and interest is certainly not unique to the Batman mythos – it is, in fact, pervasive enough that real individuals with mental illnesses or disabilities are still feared, despite the progress that science and ethics have made. What is unique about the Batman mythos in this respect, though, is the quantity and prevalence of villainous characters who are built on the archetype of the “dangerous crazy person.” I would go so far as to argue that all of Batman’s best-known villains have been Arkham escapees.
Why does Batman continuously battle the mentally ill? I have a few ideas, although I have not done formal research to determine their validity as arguments. The writers may have decided that unpredictable, bizarre, “crazy” villains would hold the readers’ interest more than average criminals, or maybe the superhero genre itself requires villains more frightening and unpredictable than average in order to provide sufficient conflict. It could also be that since Batman himself is a very peculiar character, with his all-consuming fixation on justice and his choice to wear an armored bat costume, the presence of villains who are explicitly portrayed as mentally ill and behave like archetypical mad(wo)men may make Batman seem more relatable and less frightening by comparison. Grant Morrison described the prevailing characterization of Batman in the infamously dark comics of the 1980s as “borderline psychopathic” (citation); perhaps the villains had to be taken further over that “line” if Batman was to maintain the relative role of the hero.
I know there’s a lot more to be said about the topic of mentally ill villains in popular media – again, while I think their prevalence in Batman stories is somewhat unusually high, villainous characters depicted as “crazy” are found in a very large number of narratives. If you have something you’d like to add, or if you have an opinion that contradicts something I’ve stated here, feel free to discuss it in a comment.