BRITISH COMMERCIALS DIRECTOR DAVID Slade never doubted that his debut feature, Hard Candy, would make an impact. "A number of times people asked, ’Is this going to work?’ and I said, ’Is this going to work? This is going to be like a f***ing, huge, nail bomb going off in some areas’," recalls Slade. He wasn’t wrong.

When Hard Candy premiered at Sundance last year it became what one reporter described as the "word-of-mouth don’t-see buzz movie". People were leaving the theatres distressed, confused and angry. Slade claims one man even tried to physically attack him at a post-screening Question and Answer session.

"He got up with sheets of paper he had written, and as he read he was getting more and more agitated, and eventually began to advance upon me."

The controversy has rumbled on in the US media and on message boards ever since. "I didn’t know whether to applaud or barf," wrote the New York Post’s critic. USA Today called the film "a highly original psychological thriller/revenge fantasy ... bitterly hard to take and uncomfortably intense." The Boston Globe summed things up best, however, saying: "Hard Candy is the rare movie that may be worthiest for the arguments you’ll have after it’s over." Certainly there’s much to offend, provoke and challenge here.

The film was inspired by a newspaper article found by its producer, David Higgins, about Japanese schoolgirls who used the internet to lure businessmen back to an apartment for sex, and then robbed and mugged them. "Hard Candy fits at an intersection of a number of disturbing issues of how society deals with predators, how society deals with justice, how society deals with the commodification of sexual images," says playwright Brian Nelson, who wrote the screenplay. One of the reasons it upsets people, he believes, is because "we don’t sum up the issues with a nice, little, thematic statement at the end of the film about what you ought to think".

Nelson says Higgins approached him to write something with "a character who you would think, ’Oh, I would be afraid for them,’ and actually, maybe you should be afraid of them." The scenario presented to Nelson was simple: a young girl tracks down a man, ties him up and castrates him. He wrote a couple of pages and gave them to Higgins. They were fine, except for one crucial misunderstanding: Higgins meant actual castration - cruel words were not enough. "I thought, ’Huh? Wow’," says Nelson, laughing. "I walked down the hall and said to my wife, ’This guy’s got this idea. Am I crazy to think this is pretty interesting?’ " In fact, the writer was so intrigued that he agreed to pen the screenplay on spec.

"I’M ALWAYS DRAWN TO STORIES WHERE people are faced with what they’re capable of," he explains. "Because I think that society is a big construct that we’ve erected to keep from too close knowledge of ourselves, and that’s both for good and for bad. So I like stories where people find themselves in a grey area, and the rules are up to them, and what will they find that they can really do that they didn’t think they could do before?"

A father of two daughters, Nelson says he also loves stories in which "young women get to kick some tail. I’ve been a big fan of Buffy the Vampire Slayer for years," he chuckles. "Hard Candy was an opportunity to write a really smart, teenage female protagonist."

That would be Hayley (played by the remarkable Ellen Page, who was 17 at the time), a precocious 14-year-old who agrees to meet the 32-year-old photographer Jeff (Patrick Wilson) she’s been flirting with in an internet chat room. They go back to his designer bachelor pad, continue their verbal foreplay, and knock back screwdrivers. She asks him if he has slept with any of the underage models whose pictures adorn his wall, and dances provocatively. Jeff gets out his camera. Then he blacks out.

When Jeff comes round he is strapped to a chair. "Playtime is over," taunts Hayley. "It’s time to wake up." She accuses him of paedophilia, and of possibly murdering a girl. She scours his home for kiddie porn, and discovers a stash of pictures that she says - we don’t see them - are "officially sick". Soon, Jeff finds himself tied down to a table, a bag of ice numbing his crotch, with Hayley standing over him, scalpel in hand. Ouch.

Has Hayley gone too far? If she is right about Jeff does that justify her actions? What is "too far" in such a situation? If Nelson’s Buffy reference suggests a clear delineation between good and bad, right and wrong, the film takes us somewhere less absolute. It is troubling precisely because it subverts the traditional protagonist-antagonist paradigm. Our sympathies flip-flop between the characters, and there are disturbing moments when Hayley, at least to this writer, just seems like a deluded, out-of-control product of media hysteria, and you find yourself siding with Jeff.

"This is really a film about responsibility, not the subject of paedophiles," says Slade. "The extreme circumstance of this film is a kind of little equation for how, as adults, we may or may not want to behave. Both characters are as monstrous as one another, so society dictates that one is the bad guy and one is the innocent good guy. But no, this is more like reality. And if you’re a big fan of vigilante movies like Death Wish, I think you’ll watch this and go, ’God, Charles Bronson couldn’t just go home to Chicago at the end of this’.

"It’s a really horrible, messy business, and the fallout and the echo is going to live with Hayley her whole life." Nelson adds: "Many people say, ’If I were faced with such a person, I know what should be done: we should rip their balls off.’ It’s fine to say that around the coffee table, but who’s going to go and actually do that? What would it take to do that? If we are going to be a society that takes vengeance, which maybe we have to be, what’s the cost of that?"

Hard Candy raises lots of other questions about the line between what we consider acceptable and unacceptable images of children, about the sexualisation of children in general, and about how the widespread availability of pornography on the net affects behaviour and attitudes ("It’s desensitising and changing our very DNA, I would hazard a guess," says Slade). What ripples does it have in society? Slade does not think there is anything wrong with pornography in "any specific sense", but he thinks that people should think about what it means when they look at it.

"People see it as entertainment and they’re very passive in their viewing. But, as we know, it’s a complex and often very aggressive act. Part of our subtext, I guess, is saying that just because she’s wearing pigtails, and it says on the website that she’s 18, it doesn’t mean that you’re off scot-free. You really ought to think about what you’re looking at, because there’s a lot of things involved here."

Jeff is more than just a possible paedophile - he is a man who, through his work, contributes to society’s sexualisation of teenage girls, and Hayley has had enough. "Just because a girl acts like a woman, it doesn’t mean she is ready to do what a woman does," she tells him. Jeff, to some extent, then, is representative of something we see happening on the net, in the media, in the pop world, in the film industry ("Let’s not miss one of the cores of our existence in terms of dictating iconic values," says Slade), and, as the Conservative leader David Cameron spotlighted recently, in our high street stores, which is, says Nelson, "a very unpleasant sexualising of little kids".

"The fundamental moral values, particularly in Hollywood, are deeply skewed," says Slade. "It has become almost normalised to portray women of whatever age as sexual objects - almost accepted within the structure of society - and to prime a child for that end is a dangerous thing. But it is the norm here and many other places. So if you are irresponsible as a male, and seek to exploit a young woman, society has set up the odds in your favour." Hayley, Page has said, is somebody who looked at the world and just couldn’t stand it anymore.

Slade disagrees with distributor Lionsgate’s sales pitch that Hard Candy is mainly about protecting our children, though he sympathises with their position. "That’s a bandwagon they can jump on because it’s a very difficult film to market," he says. "For me it was just a great way to open up these debates in a cinematic form."

He pauses for a moment, then tells me about a test screening in California where kids started chanting "Kill the Bitch". It was not the reaction Slade was hoping for. "But that’s the occupational hazard of making a film like this," he sighs. "You make your bed and lie in it."

• Hard Candy is released on 16 June.