A year ago, the Cannes film festival was in shock – they had just seen Heli. Which means they'd just seen puppies having their necks snapped, men being beaten senseless and a group of bored children setting fire to someone's penis. Fresh from the Riveria-ready glamour of opening gala The Great Gatsby, the crowd reacted badly – with outrage, bordering on scandal.

For his part, too, the film's director, Amat Escalante, was shocked – or, rather, appalled – by the reaction. Critics accused him of headline-grabbing gratuitousness. He accused them of ignorance, cowardice and naivety.

Fifty-two weeks on – and 51 after his surprise victory at the end of the festival, when Steven Spielberg's jury named him best director in defiance of such detractors – the shock has worn off. Yes, says Escalante, his film is violent. The critics just hadn't been primed to try to look beyond it. In his homeland, where the papers run photos of decapitated bandits as a matter of course, it barely raised an eyebrow.

"Here, nobody complained about the violence," he says, sipping lemonade in a Mexico City cafe. "People felt that it was a necessary film, a portrait of a moment. And that maybe in 10 years, they'll see this and other films and say that was something that was really happening. Hopefully, it won't be happening any more."

On reflection, what the whole hoo-hah did highlight, thinks Escalante, is a hypocrisy about the way in which cartoonish brutality is par for the course in cinema, but truthful barbarity sticks in the throat.

"If the critics analysed the first five minutes of Batman, they'd find there are about 20 people who are murdered. In my movie, there is nothing like that. It is more about how I am showing it, of course, but that for me is a much more true and honest way.

"I am a fan of gory films and I like Batman and I have nothing against any type of way of showing anything, but when you try to do something in an anti-dramatic way, in an anti-glamorous way, people get much more offended and indignant."

For him, and for the film's fans, Heli is a quiet, clear-eyed examination of the effect of Mexico's drug wars on impoverished youngsters, and how such conflict turns victims into perpetrators. Heli is a factory worker who lives in Escalante's home state of Guanajuato with his partner and baby, as well as his father and 13-year-old sister, Estela. Their lives are far from comfortable, but become untenable after Estela's soldier boyfriend stumbles across a couple of packets of cocaine, then hides them in the water tank.

"I really didn't care about the mafia or the drug cartels or any of that," says Escalante. "There are no narcos in the movie, there are only the authorities and vulnerable young people who are in trouble. But they are also the only hope. What I show is an abandonment of people – and then people abandoning themselves so that they have no moral standards any more. I feel this is happening in the whole world, a bit, but in Mexico it is very intense."

He takes another sip of lemonade. A youthful-looking 35, Escalante is a man of mild manners who saves his stridency for his work, but whose poise is unsettled by a self-questioning that seems to cut into conversation any time he feels himself getting too comfortable. Commitment to the truth is paramount, he emphasises. That's why they made Estela so young – in Guanajuato, where abortion is illegal, 13-year-old girls get pregnant all the time. It's also, he says, why he can't imagine making films with professional actors – the vast majority of the cast of Heli have no training at all.

Amateurs offer him a variety and a flexibility apparently absent from the stages, screens and acting schools of the country. Escalante speaks of preconceived ideas and of delicate egos, and speculates that his antipathy to professionals might be due to his own lack of formal training – his family had neither the cash nor the right connections to send him to film school.

In Mexico, Escalante's height and pale skin have led many to assume he hails from the country's rich and interconnected elite. And while being dismissed as "some bourgeois kid" clearly irritates him, Escalante accepts that his international background has been central to both his sensibilities and opportunities.

Escalante was born in 1979 in Barcelona, where his father, a Mexican painter from a provincial northern city, and his American mother were working. The family soon moved to Guanajuato, where he spent most of his childhood until heading for Los Angeles with his mother at the age of 12. Miserable at high school, he turned to movies, watching A Clockwork Orange every day for six months. He was then inspired by the release of Robert Rodriguez's El Mariachi, shot with a few thousand dollars and almost no crew. He bought the same 16mm camera that Rodriguez used and moved to Austin, Texas. For two years, he hardly talked to anybody as he worked in fast-food outlets, supermarkets and a video store, and spent every Tuesday night at a film club where he boned up on Tarkovsky, Fassbinder and Bresson.

Escalante had made two short films in the US by the time he became aware that a new dynamism was taking hold back in Mexico, with Alfonso Cuarón (Y Tu Mamá También), Alejandro González Iñarritu (Amores Perros) and Guillermo del Toro (Cronos). Then, in 2002, he saw Japón by Carlos Reygadas.

"Those other films were inspiring, but when I saw Japón it spoke directly to me," he says. "It was a very strong connection. It was a revelation and I had to somehow be a part of it."

Escalante wrote to Reygadas, who soon became a mentor and a friend. Reygadas helped the younger man produce and promote his first two dialogue-scarce features – Sangre, a story of sex, love and junk TV, and then Los Bastardos, about Mexican migrants in the US. Both were shown in the Un Certain Regard sidebar at Cannes.

Then came Heli, whose big win came a year after Reygadas took the same prize for his wildly obtuse Post Tenebras Lux. Both directors were confounded. Why were they suddenly flavour of the month? How had Heli won over the man who made The Adventures of Tintin? They came to the conclusion that it was a film that appealed to film-makers, noting that the jury that year had an unusual number of directors (among them Ang Lee, Lynne Ramsay and Cristian Mungiu).

"I imagine Spielberg liked the way I tried to tell the story in as visual way as possible," says Escalante. After the awards ceremony, at a cocktail party, he was too nervous to do anything other than shake Spielberg's hand and tell him that the first film he remembered seeing was ET. The truth is, he adds, that for all the Bresson and Fassbinder, the obscure Mexican auteurs and eagerness to embrace as authentic and low-budget a style as possible, his childhood was also saturated in Spielberg – endless rewatches of Jaws, a period of obsession with Jurassic Park.

"All film-makers of my age are influenced by Spielberg," he smiles. "Even if they don't like to admit it."