I am not an immoral man – as far as I can tell. I live according to some basic, reasonable and civilised (and civilising) rules and attempt to get through a day without seriously or even lightly harming anyone. On good days, I might go a small way to making people feel the world is a good place, somewhere worth inhabiting. We all should (but rarely do) acknowledge our inestimable luck at being alive right now on the only rock in the universe with living things evolving on it. We could philosophically venture into non-existence – pre-birth, post-death – but that's for some other time. Despite the whiter than white, goody-two-shoes persona I seem to have developed superficially, there is something in the darker side (sod off, George Lucas) of human nature that I find profoundly and inevitably attractive. I think I 'got' the Joker in The Dark Knight and what does that say about me?

I remember (a time of chaos, sorry. Wrong review)... I remember my first exposure to a character that came to dominate my thoughts in the late 70s. This wasn't some teenage boy's James Bond wish fulfilment. This guy was socially inept, morally skewed and had a penchant for handguns which he subsequently used to rid the world of the scum on his street corner. Was he talkin' to me? Yes, he was. There was nothing about Travis Bickle that was conventionally attractive. The protagonist of Scorsese's masterpiece, Taxi Driver, was a loner, self-condemned to wanting to right the perceived wrongs of the small part of the world he inhabited. There were glimmers in his behaviour that seemed morally driven but he was a misfit, a Vietnam veteran who'd probably seen the worst excesses of human venality and brutality outside of the confines of American society. What I had in common with this character was zero. I could have no more shot a man in the belly for pimping than I could have eaten a sprout. But I went out of my way to buy a US Army jacket (I still have it) and prowl around the mean streets (of Bournemouth, ha!) believing that the world didn't understand me and if I had Travis' guts I would have let the world know with a few very loud noises and claret a-flowing. The world would have been indifferent but that wasn't the point.

But then, justice is dull. The right thing to do with these freaks in the society we've created is essentially institutionalised zoo-keeping – lock 'em up, Fed-Ex the key to Saturn. But I think these characters, tangential to ordinariness, play a vital part in reminding us just how thin the line is between civility and barbarism. The Joker is the bloodlust that ferments when we've been cut up by another car, the rage at the salesperson's cold call, the incandescent fury that switches on when we know we've been conned by a multi-national. We're all freaks if that's the case. But some of us are more equal freaks than others.

There is a book called The Profession of Violence and it's a terrific read. It's about the reign of the Kray twins in 50s and 60s London. If the name 'Kray' doesn't register, the Kray brothers were powerful thugs with a soft spot for celebrity and their own mother. I won't go into the sordid and shocking psychoanalysis that revealed that one of these ultra-tough men was bisexual and a paranoid schizophrenic to boot but I will say this. The twins, engaged in any violent act, were simply not afraid of getting hurt. This is significant because it places the foundation of power firmly in their laps. It is the reasoning of the suicide bomber – total self-belief as a tool of destruction. Ultimately, the basest and cornered human being acts because of two unwavering things – fear of violent reprisals against themselves or family/friends and the desire for money. That's faintly depressing as a fact right there, the money part most of all. This is one of the reasons I enjoyed the Joker's setting fire to his half of the cash earned by being utterly indifferent to what holds most of us in thrall. I suspect a scene was deleted as that money pile burns very rapidly and sitting atop was Lau, the Chinese mob accountant, identified earlier as a squealer by the Joker. I couldn't imagine the scriptwriters letting go of a well signposted quip. As the money burns, Lau would literally squeal as he burns with it.

This is why Batman's foe's latest incarnation is so mesmeric, and dare I say it, astonishingly attractive. He doesn't care for money and invites violence against himself as a strategy to turn good people towards committing bad acts. His motive is to show the operating system of the naked ape as the same one of the ape we have evolved from. When driven (by loss, violence or stark choices) we are ultimately enormously selfish. In the movie, the decision taken by a convicted felon elicits a cheer. He's a villain but the only man in a tense situation with the right answer to an impossible choice. In some ways he's an independent torch light in a room in which the Joker has cut off the power. This is human nature asserting itself against a man who opens the floodgates of chaos.

The Joker is a man who has breezily abandoned morality. He has accepted a life of chasing cars without knowing what to do with one if he caught it. He is, as he says "an agent of chaos." His method is to break humanity, to tip it into the startling recognition of the animal instinct in all of us with just some barrels of gasoline and a few bullets. The veneer of civilisation is so easily discarded. That sentence will come to haunt my son in a few decades when oil becomes as scarce as gasoline as in Mad Max II: The Road Warrior. In Griff Rhys Jones' two BBC documentaries on anger broadcast last week there is some appalling CCTV footage of people (granted, fuelled by alcohol) perpetrating shocking violence having lost all semblance of what we humans prize so highly – reason and civility – that which separates us from the beasts of the field.

The Joker doesn't seem to betray fear even when apparently falling to his death laughing maniacally as he plummets knowing that his moral mirror id, Batman himself, will not break his only rule – not to kill. Yes, The Joker is human. He feels pain. His intelligence is vast and not questionable and if he ever met Sigmund Freud he'd give the vaunted psychoanalyst nightmares for the rest of his life. In The Dark Knight, the Joker reveals the origins of his scars in two entirely conflicting accounts and in so doing makes a mockery of the analytical process that seeks to define such men. Understanding the Joker is not the point. So why in the movie does almost everything he says ring so true about the human condition?

We have to heap praise on the screenplay by the director Christopher Nolan and his brother Jonathan. We also have to acknowledge that the actor, the tragically late Heath Ledger, was directed by Nolan and that his performance, as truly inspired as it is, is still a collaboration. Michael Caine remarked that one of Ledger's bits of business (the accepted term for an actor's – or character's – idiosyncrasies) was one of sublime invention. It's the small moment when he attempts to control his hair upon seeing a beautiful woman at a fundraiser for the whiter-than-white lawyer Harvey Dent. The idea that such a simple preen could obliterate his incipient violence and ghoulish appearance is utterly inspired. There's also a small moment of sublime satisfaction nestled in the key action scene of the film. Heroic lawyer Harvey Dent is being escorted across town while the Joker attempts to catch or kill him. Batman intervenes by driving the Batmobile underneath one of the attacking vehicles, in this case a garbage truck, and pushing it backwards. I must just acknowledge the wonderful miniature work in this sequence. But the small moment I'm referring to is the Joker's acceptance that (a) Batman would intervene, that (b) he couldn't have forseen Batman achieving this manoeuvre and that (c) he shows a kind of respectful surprise in the middle of his own mayhem. Ledger does it with a throwaway look and the simple exclamation of surprise in the single word, "Hmm..." Wonderful. There are reasons that The Dark Knight's box office is knocking on the billion dollar door. This extraordinary performance may be one of them.

Ledger inhabited the role in a way so few actors can. I am racking my brain to put another actor in his place, one that would be as chilling, charismatic and convincing. On paper, the design of the character's make up and the tongue snaking out every few seconds, presumably to draw attention to the scarred face, may seem like a recipe for disaster. As fellow reviewer Lord Summerisle said in his review of the movie, "The Joker never looks ridiculous or silly..." In the reinvention of the role, Nolan and company took risks. Gone was the plunge into a vat of acid that is the well established origin of the literary (and the Burton movie character's) Joker. His face is a mutilated one, scarred of cheek, made striking by some extraordinary make-up that makes the actor damn near unrecognisable. In the bank heist that opens the film, we get a stunning first close up of that striking face accompanied by the line "I believe that whatever doesn't kill you simply makes you stranger..."

But what really sets the character off is the 'magic trick'. This is murder as a whip crack and respect earner. This abnormal creature sits in front of all the mob bosses who own Gotham through pay offs and corruption. He's just stolen all their money. One of the assembled crime-lords simply wants to kill the guy who dared to emasculate the underworld. The Joker drives the mobster's associate's head down on an upturned pencil, killing him in a second. "Ta da!" The pencil's disappeared but a warped and dark respect has flowered in its place grown from the death throes of an uncared for henchman. It's a casual murder, a shocking murder but it sets up the Joker as a serious and deadly threat instead of the bank robbing thug witnessed in the opening scene. The 'magic trick' is the kind of violence that almost makes you laugh. But get this. Whenever Nolan cuts to Ledger, the Australian actor commands the film. I felt a little sorry for Christian Bale whose Batman the movie is cheering for but if ever there was a movie that proved the bad guys get all the best lines, this is it. Bale is a terrific actor but he's up against a character whose essence is what lies at the heart of a human being stripped of morality. And let's face it. We as an audience can subconsciously recognise that the Joker is all of us, at heart and in soul. Scary doesn't begin to cover it. Yes, we are civilised but what does that mean? Those of us who are in on 'the joke' may not be able to help smiling knowing how easily our masks can slip – or in Gotham's case, be yanked off by a deranged clown.

Ledger's previous performances hinted at 'the Jeff Bridges Syndrome' – an immensely talented character actor cursed with leading man looks. Those looks have been replaced in The Dark Knight by a terrifying mask of white powder and a smeared blood-red gash of a mouth. When I first saw photos of the character in a film magazine a year ago, I remember thinking "Jesus, what is this?" I had to sit down and watch Gilliam's Brothers Grimm to remind myself what this guy actually looks like. Even in fluff like A Knight's Tale, Ledger is commanding but as the Joker he's the maypole around which everyone else dances. The dance (of death) is a powerfully unassailable one and if the Academy doesn't honour this extraordinary performance next year then it's time to see how expensive gas and a few bullets really are. No, that's not a threat. It's a joke...