I feel that to have an effective criminally insane villain, certain things are necessary:

1. The writer should have a clear idea of the character’s mental issue, how their perceptions and thoughts are abnormal (more so than simply “they are chaotic”).
2. These mental issues make the person not responsible for their actions as they are perceiving the universe too differently.
3. There’s a tension in the story because of the character’s unreliability. It is hard to deal with this character because the villain’s perspective is so displaced from what’s real. If he has henchmen, then there probably should be a moment where the henchmen are uncertain how to interpret their boss’ commands (unless the henchmen have bought into the issue).
4. The hero’s confrontation with the villain must have internal tension, a sense of “how do I deal with this person?” The villain is ultimately not responsible, but still must be stopped. The story must feel a sadness that such a violent response against the villain is necessary.
5. Whatever the villain’s final fate, there must be a sense of tragedy. If the villain cannot be cured, then there must be sympathy for the villain, that he or she is basically a prisoner in their own head.

For many people (myself included), Batman the Animated Series from the 1990s is considered one of the greatest takes on the character, a revolutionary cartoon that reinterpreted previously lame villains (such as Mr. Freeze and Mad Hatter) and added several compelling new characters (such as Harley Quinn and Renee Montoya). It did, sadly, carry on many issues from the comics, including numerous characters classified as being insane when they had no right to be, Arkham Asylum as almost more of a prison than a hospital, etc. But that said, it did feature a few characters who were legitimately criminally insane, and they were some of the most compelling episodes.

Ventriloquist

He was created in the late 80s, and has the interesting distinction of being the most intriguing Batman villain created when I was alive. Arnold Wesker is a shy withdrawn man who has repressed all his anger, frustration, and aggression. An accomplished ventriloquist, he expresses his negative thoughts through a dummy named Scarface, who Wesker is convinced is a totally different person. Scarface becomes a ruthless gangster who treats Wesker as his pathetic valet, constantly denigrating him in public, and Wesker believes that they are two entirely different people.

He’s got a clear mental issue – dissociative personality disorder brought on by repressed anger, and he clearly his consciousness is not responsible for his actions. As far as his consciousness is concerned, the Ventriloquist is an innocent man held hostage by a villain, and whenever anyone confronts him, he sincerely announces that (“It wasn’t me! It was Scarface! He’s the one you want!”). After being persecuted by Scarface, Catwoman attacks the Ventriloquist and is about to claw his face, and when the Ventriloquist begs for his life, our sympathy is with him – he think he’s innocent. There is also a perfect moment of unreliability in his first appearance on the cartoon, where Batman convinces Scarface that Ventriloquist has betrayed him, and Scarface demands that his goons execute the Ventriloquist. The goons, of course, hesitate, as they know that killing him would also kill their boss, and Scarface is enraged at this “betrayal.”

Perhaps most tellingly, the doll Scarface is destroyed in the climax, and then we see Ventriloquist at Arkham Asylum some time later. The doctors are happy that he seems to be making a full recovery. But then we see he is secretly making a new Scarface doll. Usually when there is a scene at the end of a Batman episode that shows the villain will return, the emotion is menace – the villain will escape and cause suffering. But here the emotion is tragedy – not that the Ventriloquist will hurt someone (though he likely will do that too) but that he is not free of his mental issues.

Maxie Zeus

One of the most peculiar of Batman villains – Maxie Zeus thinks that he is the Greek god Zeus and he fits everything into this delusion (such as thinking of Batman as Hades). Probably no villain is as disconnected from reality. He is often ignored in the comics or treated as a joke because his delusion is so ludicrous – but that’s what makes him truly insane.

The Animated Series episode with Maxie (“Fire from Olympus”) is a powerful and strange story because it truly explores what makes an insane villain so dangerous and yet so sympathetic. Maxie Zeus is accompanied by two henchmen and his girlfriend Clio. Clio is a compelling addition because she is his loved one viewing his fall from insanity, begging him to return to lucidity. She stands in for numerous people who witness their loved ones fall from reality, and desperately try to help them without knowing how.

Maxie’s most prominent crimes are ones that seem so reasonable in his own world. He destroys police zeppelins that get close their tower merely because “mortals are not supposed to approach Olympus,” attempts to murder people who renege on deals with him because they have “shown disrespect to the gods.” His actions are so divorced from reality that it is impossible not feel sympathy for Maxie and for Clio. And tellingly, Batman convinces her to help defeat Maxie because Batman claims he can get the man the help he needs.

Then, when Clio tries to make him see reason, we have the perfect moment of unreliability. He believes her actions are because she is “merely” a demigod. He chains her to his lightning machine so that the “lightning of Zeus” will purge her of her mortality, making her a full goddess at Maxie’s side. His goons start to question him, just like the Ventriloquist’s men did.

And then, we have the ending – which manages to be both funny but also very sympathetic towards Maxie Zeus. He’s in a straight jacket, being rolled through Arkham, and is utterly happy, because he mistakes the Joker for the trickster god Hermes, Poison Ivy for Demeter, and Two-Face for Janus, and is convinced that finally he is in Olympus. The final image is not a villain who we can be happy is incarcerated but a poor victim trapped in his head.

Other Villains

There are a few other villains in Batman that are clearly insane. For example, Killer Croc is often so animalistic he seems almost entirely motivated by instinct; it’s hard to blame someone for his actions when he just starts screaming “Hungry! Pain!” Humpty Dumpty is a savant great at taking apart machinery and fixing it, and he eventually decided that the reason his grandmother was so abusive was that she must be broken – so he took her apart to fix her. But they’re rare. Certainly most of the more prominent characters do not feel insane at all.

What would they be like if they were actually crazy?

Two-Face

Two-Face is almost there. He clearly has an issue that takes away control of his actions, he often recognizes he has issues, and he has loved ones grieving for him, and Batman clearly considers him to be a damaged person to be helped rather than a criminal to be punished. However, he falls apart a little because of the lack of specificity of his mental problem. He’s frequently referred to as having multiple personalities, but that is very rarely represented – and seems more like an attempt to just add more duality imagery to him than a thought about how he should be perceived. Perhaps more importantly, insanity means being so separated from reality that you cannot judge right and wrong, and Two-Face’s issue is he feels a need to flip a coin to decide his actions, but the coin flip is always between “good” and “evil” actions – if he is able to separate what makes an action good and evil, and he uses definition that regular people do, then he clearly does understand right and wrong – he just chooses to ignore it.

For Two-Face to be truly insane, his coin flip should not be about whether to be good or evil, but about deciding what the “good” choice actually is. Studies have shown that any choice we make is ultimately with our emotions, our “gut” — if the emotional part of the brain is damaged, then no matter how much reasoning the person is still capable of, they will be unable to make proper choices. If part of Two-Face was damage in whatever scarred them, then he could potentially have no ability to judge his own actions. That would present someone highly sympathetic and clearly dysfunctional, a person robbed from any internal sense of meaning.

One problem with that is that it does separate Two-Face from the iconic-ness of his coin, which represents the choice between good and evil. It is possible that it could still often represent the violent vs non-violent choice (“do I shoot this person or not”) — but frequently which choice should be made with the scarred side is arbitrary (“do I rob this store or the other?” “Do I shoot Batman or Robin?”). It would dilute the purity of Two-Face’s iconography, but would make his situation more philosophically compelling, and ultimately make him a more sympathetic and accurately insane character.

Poison Ivy

Poison Ivy is a seductive woman with an affinity with plants, and an eco-terrorist ideology. She is an iconic Arkham inmate, but what exactly is her insanity? Caring about the environment is not insane – neither, for that matter, is killing people over it – though it is, of course, criminal. To be insane she would need a harsher separation from reality, and more confusion on why others don’t see things her way.

Giving her a warped sense of empathy could work. She has an easier time feeling empathy for plants than for humans. She thinks back in sadness at when her grandparents’ house burned – not because the grandparents died but because their rose bush did. Ivy can’t understand why other people are so cruel to plants. If one goes this route, it would make sense for Ivy to only eat the parts of plants that doesn’t harm them – she eats fruit and nuts, but never roots. She feels about eating carrots the same way that most people feel about eating human flesh (or at least monkeys). Ivy is earnestly baffled why she’s considered a villain for murdering a CEO whose company clear-cuts.

Harley Quinn

Harley suffers from the association of chaotic behavior with insanity. Of course she’s insane; she’s so weird! But then what exactly is her craziness? Harley is at her most frightening when she is bubbly and goofy while doing terrible things. There’s a bit where she has Catwoman strapped to a conveyor belt, and is about to turn her into catfood. Harley is joking, and seems to honestly think that Catwoman will appreciate that in death she’ll be helping some cats.

Harley does not take life seriously. She treats it like a game. What if that’s what she literally thought life was? That it was just fun, with no serious consequences – like an old Bugs Bunny cartoon. People may fall off buildings, be set on fire, but in the end no one is really hurt. There are no consequences. It would make her bubbling personality very dangerous.

Mad Hatter

Few Batman villains have changed more dramatically than Mad Hatter. First he was just a villain who looked like the Alice in Wonderland character, then someone obsessed with hats, then someone obsessed with Lewis Carroll, then someone whose obsession moves between hats and Carroll depending upon the author. He often feels like a character who was declared insane by the authors simply because he has “Mad” in his name, and then made to giggle and quote Alice more in order to feel crazy without any real thought to what exactly his issues are.

Of Mad Hatter’s two obsessions, the Carroll one is far more interesting than hats, and easier to build an insane outlook on. The Alice books are full of bizarre outlooks, logic puzzles, and weird philosophy, and it is easy to build an insane perspective from them. The most effective Batman stories with the Mad Hatter play up the dreamlike subjectivity of the Alice books, producing a character who doesn’t really believe that the world is real – it’s all just something dreamed by the Red King (or possibly Mad Hatter himself). A world full of all sorts of backwards logic, including that the best way to go over a gate is to stand on your head and that words can mean whatever you want them to mean. The Mad Hatter is a person who believes he has stepped through the mirror into Looking-Glass Land, a realm where flowers talk, rabbits have waistcoats, and everything is a dream. When he becomes a man suffering from delusions, hallucinating his dream realm, he becomes a far more compelling figure.

Riddler

Riddler is a man obsessed with proving he is smarter than the smartest man (who he thinks is Batman), and so he constantly leaves riddles, forcing Batman to try and solve them. In later stories, they have played up the riddles as a compulsion, that he finds himself unable to not leave them behind. This can clearly mark him as abnormal, but crazy? No. It’s the sort of thing that might get a psychiatrist to visit him in prison but not get him sent to a hospital.

It would be hard to reinvent the Riddler to make him actually insane, as his defining trait beside riddles is his total lucidity. He very clearly understands his environment, and knows the laws, and how they can be played with. The only way to make him insane while keeping that cunning is to make him some sort of solipsist who believes that no one is actually real except him and Batman. It would explain why he is so obsessed with Batman, and why he doesn’t care if other people are hurt as part of the schemes. Unfortunately, it would also make his issues pretty similar to Mad Hatter’s – a genius who basically believes the universe is a dream operating under its own logic. Honestly, I feel Riddler would be better served as being reclassified as abnormal but sane.

Scarecrow

The Scarecrow. A psychologist who dresses up like a Halloween decoration and is obsessed with spreading fear. In some stories, he is spreading fear just for the hell of it, but in others he seems to be doing some psychological experiments. What is his insanity? It’s something that is never really established. He is creepy and obsessed, but besides that there really isn’t much.

Is it some extreme megalomania combined with delusion? Sometimes the way he talks, he almost sounds like he thinks he’s some sort of god of fear. Is it that he prioritizes scientific research so much that he cannot understand why people are shocked by his actions? That sort of mad scientist approach could be a direction to go. Possibly he might even think that people facing their fear, even if they die, is what they need to do. Though admittedly all of these move him away dramatically from his normal personality. He is so clinical and yet sadistic that it is difficult to reinvent him in a way that seems legitimately crazy.

Joker

Batman’s arch-enemy. When one asks “Is the Joker crazy?” Most people would answer “Of course, he is. He’s the Joker!” What then is his mental issue? Sure, he’s weird and wild, and does all sorts of funky things, but what exactly is his problem? It’s something that surprisingly few people are interested in exploring – probably partly because one big element of Joker’s appeal is how mysterious he is. However, I feel that if you are going to present an insane character, then it is important to explain how he is insane. Otherwise, go another direction with him

Grant Morrison suggested that Joker’s brain has difficulty processing sensory information, so that he is constantly interpreting it in a different way. That’s used to explain why different stories have dramatically different takes on him – joyless assassin, goofy thief, giggling serial killer, etc. Others (including Alan Moore) have suggested that Joker is such a complete nihilist that not only does he feel the universe is one big pointless joke, but he believes then that the only moral imperative is to teach others of this cold hard fact. The world is chaos, and everyone should know it. Or, in a more light-hearted way, he just sees everything as a cartoon, just like Harley does. In this case, the one moral imperative is to get everyone to laugh.

Unfortunately, a lot of people also like to think of the Joker as one of the DC Universe’s ultimate embodiments of pure evil. If he is insane, then by definition he can’t be that. Then Batman is an unfeeling brute beating up a man not responsible for his actions. This is an interpretation many people are not comfortable with. Fair enough, but in that case don’t call Joker and his ilk crazy.