One of the many things I appreciated about the recent Batman film, The Dark Knight (2008), is how the Joker's background and raison de'etre are kept appropriately mysterious and vague. In two separate scenes, one with the gangster Gambol and one with Rachel Dawes, the Joker reveals the origins of his facial scars. The two stories, of course, are at odds with each other - suggesting that neither is true. Herein lies the horror of the Joker: he refuses to provide us with an easy explanation for his existence. If he was understandable, perhaps just "insane" from some traumatizing childhood experience, we could package him away into a safe little box along with Two-Face, Scarecrow, et al. and reduce his existence to consumable ideas & theories.
Another possibility, often cited, is that the Joker is Batman's ethical doppleganger. If Batman did not exist, neither would the Joker - and as long as Batman haunts Gotham, so will his evil twin. The Joker is fond of taunting Batman with this dilemma, mostly to piss him off. Sometimes this version of the creation myth is made more literal by having Batman be the cause of Joker's disfigurement. This is the storyline revealed in The Killing Joke (1988) by Alan Moore, one of the first graphic novels I ever read. The Joker starts out as a two-bit comic trying to make ends meet for himself and his pregnant wife:
Odd to feel sympathy for the Joker, no? But you gotta love that bow-tie.
In increasing desperation, he falls in with the wrong crowd, unknowingly taking on the role of the Red Hood in a doomed robbery attempt of the Ace Chemical Processing plant. Batman shows up, and "the Joker" (at this point unnamed) jumps into a vat of chemicals to escape.
His skin burns and discolors, and he emerges as Bolland's archetypal Joker:
If you've only seen the movies, you may recall that Tim Burton's version of the Joker borrowed heavily from this myth - providing a relatively straightforward case for why the Joker hates Batman so much.
But this is a little too convenient and doesn't offer the Joker any of the psychological depth he deserves. In the end, it's not any specific trauma that creates the Joker, it's not the Batman - it's us. It's this crazy, fucked up world. Here's the Joker's monologue to Batman (but really, to us), explaining it all:
I mean, what is it with you? What made you what you are? Girlfriend killed by the mob, maybe? Carved up by some mugger? Something like that, I bet. Something like that...
"Something like that happened to me, you know. I... I'm not exactly sure what it was. Sometimes I remember it one way, sometimes another... If I'm going to have a past, I prefer it to be multiple choice! HA HA HA!
"But my point is... My point is, I went crazy. When I saw what a black, awful joke the world was, I went crazy as a coot! I admit it! Why can't you?
"I mean, you're not unintelligent! You must see the reality of the situation. Do you know how many times we've come close to world war three over a flock of geese on a computer screen? Do you know what triggered the last world war? An argument over how many telegraph poles Germany owed its war debt creditors! TELEGRAPH POLES! HA HA HA HA HA!
"It's all a joke! Everything anybody ever valued or struggled for... it's all a monstrous, demented gag! So why can't you see the funny side? Why aren't you laughing?
Ironically, it's more sobering to consider the possibility that we, as a society, have created a breeding ground for evil men. When we see the big bad black man being carted off to prison, we want to believe in simple explanations for human criminality and violence: bad neighborhood, bad parents, bad schools. But when we're forced to ponder why those bad neighborhoods exist in the first place, when we begin to consider our own personal role in the propagation of inequality and hate, our self-righteous indignation might well fade into uncomfortable laughter.
Like many other fans of the genre, I find the Joker to be a particularly interesting and even appealing villain. Ultimately, maybe it's because we sense that he's just smarter than the rest of us. He's decided he doesn't want to play the "civilization" game anymore (I loved the scene in The Dark Knight when he burns that giant pile of money). He's ripped off the rose-colored glasses and sees the world in all its ugliness. He respects the Batman more than us, because at least Batman has ideals and values - which, though absurd and meaningless, are worthy of admiration. But he has no sympathy for the sheep and wishes we'd all just get a little perspective.