Character Analysis
We hear a couple of seriously gruesome stories about the Joker's origins in this film, all delivered by the man himself and all of them complete poppycock.
If I'm going to have a past, I prefer it to be multiple choice," he tells Batman in The Killing Joke, which is a sneaky way of saying we don't know who the heck this guy is. The authorities can't figure out where he comes from, we never see any secret hideout or scummy apartment belonging to him, and friends? As Maroni puts it, "have you met this guy?!"
All of which is a big way of saying we have no idea who the Joker is or how he got to be, um, that way. We only know him through what he does, and a guy this sick and twisted tells us volumes by his actions. We first see him robbing a bank, but even there, he seems to be about more than just a chunk of change. He's ripping off a mob bank, for starters, and as we eventually see, he's happy to light his giant piles of money on fire once he has them.
Then there's the fact that the entire robbery seems to serve as some kind of ultra-violent performance piece. He induces all the other robbers to kill each other off, then reveals himself as the last man standing as a sort of "ta-da!" to the hostages in the bank. You can almost hear him yelling "Get it? IT'S A JOKE!" to his unwilling audience leaving those dead bank robbers laying hither and yon as a sick little punchline.
Chaos
The intention there, as it is throughout the film, is to cause a little panic and fear. He tells Two-Face,
JOKER: I'm an agent of chaos.
He really, truly means it. Rules bother him a lot, which means that doing anything predictable or expected is an absolute no-no. And there's more than freelance anarchy going on there. He hates rules because he sees how often people break them: how little things like morality and civilized behavior are chucked out the window when people start to get really scared.
In short, he's on a mission… a mission to show the people of Gotham City that they are every bit the monsters that he is. He pushes them into abandoning every principle they hold, he shows them how little the police or the Batman can do to keep them safe, and then he sits back and waits for the city to tear itself apart.
That guides all of his actions: not money or power so much as turning Gotham City into one big lunatic asylum. He wants to drag them down to his level, to show them how horrible they can be when they get pushed. It never happens, of course, but oh my does he come close. In that sense, he really is a terrorist: he doesn't have any goals that anyone can understand beyond anarchy for anarchy's sake.
And we're not really sure how he makes his plans. We don't see him marking up a big map of Gotham, or chortling with his henchmen the way they did back in the Adam West days. His schemes are pretty complicated, and seem to involve a lot of preparation beforehand (those charges ain't gonna set themselves). On the other hand, planning sounds like something that really annoys him, to the point where he vehemently defends it in his argument with Two-Face.
JOKER: Do I look like a guy with a plan?
He grins sarcastically, and considering that he's wearing a nurse's skirt, it's hard to argue with him.
If it's true, that would mean that he's some kind of idiot savant, connecting the dots in the strangest way possible and riding the ensuing tidal wave like the Big Kahuna himself. He's got a general sense of the chaos he's going to cause, and an ingenious way of doing just what he needs when he needs to in order to destroy any semblance of civilization around him.
That's the likely answer, but like a lot of things about the Joker, we're never going to know for sure whether he really is a spur-of-the-moment guy or whether his argument with Harvey was just more hot air. In the end, it probably doesn't matter. It only matters how wildly successful he is at bringing everything—everything—thundering down around everyone's ears.
Mob Mayhem
He doesn't leave the bad guys out of the fun either. They're an organization, after all, and you've already seen how he feels about organization. When he first crashes their "little group therapy session," he arrives like the Devil himself, there to collect all the naughty little children and take them away.
His contempt for the underworld springs both from his disdain for their goals, and for the fact that they think they can control things the same way Dent and the cops do.
"This city deserves a better class of criminal," he sneers at one of the city's big crooks, and he really means it. The havoc he wreaks with them, to the point of burning down a giant pile of their money, doesn't leave them with anywhere to stand. They're in disarray just like the cops are, leaving panicked crowds and one very cranky Batman in the place of structure and order.
That, in the end, is all he really wants. And if he ever learns what Bats and Commissioner Gordon have to do to stop him, he'd probably just laugh his creepy little head off. The only thing that keeps him from total victory is Batman himself: who proves that Gotham has a soul and, against all odds, never breaks his one rule.
And the Joker is so twisted that the idea actually fills him with glee. In his own sick way, he kind of loves Batman. He even says so in the interrogation room, where he mocks Tom Cruise's "You complete me" line from Jerry Maguire. He can't imagine life without Batman to test, over and over again until one of them cracks. "I think you and I are destined to do this forever," he cackles at the very end, suggesting a lifetime of Batman thwarting the Joker's latest apocalypse by the skin of his teeth. That's about the scariest thing we've ever heard of, and we're betting the clown couldn't be happier about it.
Posthumous Victory
We'd be remiss not to mention the fact that Heath Ledger—who won an Oscar for his role in the film—died before the movie came out. Whether or not that helped the film ratings is debatable, but, uh…it did.