The Cannes Film Festival moved into controversial territory Thursday and Friday, with screenings of the school massacre tale We Need to Talk About Kevin and Unlawful Killing, about the late Princess of Wales.

Based on the bestselling book by American-born, British-based writer Lionel Shriver, We Need to Talk About Kevin is about a teenager who commits an atrocious crime.

Tilda Swinton has the key role as Kevin's mother, Eva, who is numb with grief and guilt. She constantly reviews her son's childhood for clues about where she went wrong.

"The whole film is about guilt," said British director Lynne Ramsay.

Ramsay, 41, first wowed Cannes in 1999 with her debut feature Ratcatcher and in 2002, brought Morvern Callar to the festival.

'It's certainly a very bloody business being a parent, and it's a really bloody business being a child'—Tilda Swinton

She says her latest film is not violent, especially by Hollywood standards, but is "a psychological horror film."

Swinton is drawing praise from critics for her portrayal of the haunted Eva.

"It's like a nightmare scenario, but it's not that far from the everyday experience of being a parent," she said at a press conference Thursday.

"It's a bloody business, having a family," said Swinton, the mother of teenage twins. "It's certainly a very bloody business being a parent, and it's a really bloody business being a child."

Favourite for best actress award

The Los Angeles Times published one of many reviews calling Swinton a favourite for the Cannes best actress award, hailing her "raw yet controlled performance."

London's Independent newspaper took issue with the portrayal of Kevin (by actor Jasper Newell as a child and Ezra Miller as a teen), saying he is "one-dimensional" and "menacing but unbelievable."

But Variety called the film "an exquisitely realized adaptation" of the novel, saying Ramsay "lets pure film technique do the heavy lifting in order to convey the desolate emotional climate."

Co-screenwriter Rory Stewart Kinnear — who is married to Ramsay — said the film goes where many others fear to tread.

"I think the idea of a mother not loving her son is one of the last taboos, and something people don't want to talk about," he said.

Unlawful Killing screened

Keith Allen, father of pop singer Lily Allen, directed Unlawful Killing, a documentary that explores conspiracy theories surrounding Princess Diana's death. ReutersOn Friday, Unlawful Killingscreened for buyers and journalists on the fringes of the festival. Keith Allen's film revisits conspiracy theories surrounding the death of Diana, the late Princess of Wales.

The documentary has been criticized in Britain for including a picture of Diana after the crash — images the Royal Family successfully suppressed at the time of her death.

It also depicts Prince Philip as a psychopath and the British Royal Family as racist "gangsters in tiaras" who hated Dodi Fayed, the man Diana was dating before her death.

Allen says the film is "forensic" and shows that the British and French authorities conspired to kill the couple and cover it up.

Partial financing for Unlawful Killing came from Fayed's father, Mohamed Al Fayed, owner of Harrod's. Al Fayed has long accused the Royal Family of a conspiracy to murder the couple.

With files from The Associated Press