^ In the Joker's mind perhaps all of them are.
Exactly true... it's a common theme in the books (and part of what defines who the guy is). Even HE doesn't know when what he's saying is truth or when it's delusion.
The best version of his origin was given in "The Killing Joke" (IMHO). They tried to to a riff on that during the Burton "Batman" but they totally screwed it up, AFAIC.
The short form... the guy is a chemical engineer with a young wife and a newborn baby. He's always been a fairly funny guy, even done a little on-stage "standup." He loses his job, though... and can't make ends meet. He tries to get a standup job to bring in a little money but (due to his depression) isn't funny at all ... nobody "gets" his jokes, nobody laughs. However, there are a couple of thieves at the club who see his desperation and decide to take advantage of it.
They bring him in to help them rob the very chemical company where he previously worked. These two guys have a scam... they dress up their "Patsy" in a costume, the "Red Hood," and if they get arrested, they claim to be minor players in the "Red Hood Gang" but in reality they kill the guy they'd brought in as the "Hood" after each caper so they get to keep everything.
He's desperate... so he agrees (not knowing the "rest of the story" regarding their scam obviously). But just before, he gets cold feet, and tries to back out. And at that point he discovers that an accident with a baby bottle warmer... his house burnt to the ground and his wife and infant child both died. He's then threatened with death if he backs out...
During the break in, the (as yet still new to the city and "mythological") Batman shows up. The guy freaks out, seeing a demonic figure... and flees. But he falls into a vat of chemicals (which, naturally, are being dumped into the river... remember, it's a comic, not reality!)
So, he comes to on the edge of the river, chemically altered (both in mind and body) and psychologically fractured... and the Joker is born.
The character is filled with delusions, hundreds or thousands of "origins," he remembers... all different... but all with the basic idea that "life has a perverted sense of humor" that matches his own. As far as he's concerned... he is who he is because that's what LIFE is like... and he's the only one who actually GETS THE JOKE.
It's less a matter of him being mean and vicious, and more a matter of him seeing this as how the universe really is, and all our ideas of morality and social consciousness and so forth are the REAL delusions.
Of course, he fixates on the Batman. Over his career, he kills thousands, and one of the issues that weighs most heavily on the Batman character is the idea that had he KILLED the Joker rather than just capturing him and incarcerating him over and over, all those who had died would still be alive. But, and this is KEY to the Batman's character... THE BATMAN DOES NOT KILL.
They got this right in the Nolan movies... and that's not by accident. They know that he doesn't kill, and they've honored that (this was my biggest complain re: Burton's take on the character!)
And THAT is the main reason that I think that the ending of "The Dark Knight" is a setup for another sequel. They know this about the character and honor it... and I don't believe that they'd have done this... and created such a MASSIVE conflict for the character... unless they had a REASON for doing so.
