There is a heart-stopping moment in a new documentary about the survivors of Chadian dictator Hissène Habré’s torture chambers, when one of the torturers kneels down in front of his victim and begs for forgiveness.

“I had to follow orders,” mumbles the man, now living on the streets as an outcast. “Then why did you have to beat me so badly?” his victim asks, handing the former gendarme the rubber pipe he used to flail his prisoner’s leg to a pulp. “Your superiors told you to stop, but you went on and on,” adds the victim, who lost a leg as a result of the beating.

The scene is typical of the muted but unflinching encounters that fill Hissein Habre, A Chadian Tragedy, Mahamat-Saleh Haroun’s film about one of Africa’s least-known mass killings, which premieres at the Cannes film festival on Monday.

Some 40,000 people were murdered during Habré’s eight-year reign of terror between 1982 and 1990, while the west looked the other way, more worried about the cold war and Muammar Gaddafi in Libya.

Habré was an ally of the Americans at the time. French money even paid for the country’s political police, the feared DDS, who committed torture on an industrial scale, according to Clément Abaifouta, who leads a survivors’ group in the capital N’Djamena.

Abaifouta’s group has spent 15 years trying to bring the former rebel leader – who was deposed in 1990 – to trial. Habré will finally be judged later this month at a special tribunal in neighbouring Senegal, where he had fled into exile.

One of the victims featured in the film, Adimatcho Djamai, who was tortured so badly he spent more than two decades forced to lie on his back, died the day he was due to testify at Habré’s trial.

Haroun said he wanted to cast a light on what he calls “this genocide” largely ignored by the outside world “because it was some business of the blacks” carried out behind closed doors.

The director uses Abaifouta as his narrator, visiting his fellow survivors and gently coaxing the horrific stories of their torture from them.

Abaifouta says he would sometimes wake to find another inmate dead beside him and “be glad that it meant a little more space. That is what we were reduced to” he said. “We were beasts.”

Haroun said most of the people who were rounded up by Habré’s DDS henchmen “were innocent. They were arrested for no reason, the random victims of a bloodthirsty regime.”

Haroun – Chad’s foremost filmmaker whose film Grigris competed for the Palme d’Or at Cannes in 2013 – said he wanted to see “if was it possible to still live together after such monstrosities. Can survivors still find a place for forgiveness in their hearts?”