Nuon Chea, regarded as the regime's chief ideologue, showed no emotion as the short video clip, dating from 2001 or later, was shown on the second day of opening statements at his long-awaited trial alongside two co-defendants.
International co-prosecutor Andrew Cayley told the UN-backed court in Phnom Penh that the trio, the three most senior surviving Khmer Rouge members, were "thieves of time" and the "common murderers" of a whole generation of Cambodians.
"Brother Number Two" Nuon Chea, former head of state Khieu Samphan and ex-foreign minister Ieng Sary all deny charges of war crimes, crimes against humanity and genocide over the deaths of up to two million people during the Khmer Rouge's 1975-79 reign of terror.
Prosecutors showed the packed court the footage of Nuon Chea, taken from the 2009 documentary "Enemies of the People", to support their claim that the movement had a policy of killing enemies and those it regarded as disloyal.
"If these traitors were alive, the Khmers as a people would have been finished so I dare to suggest our decision was the right one," Nuon Chea calmly tells a Cambodian journalist in the clip, adding "If we had shown mercy to the people, the nation would have been lost."
The footage was shot at some point between 2001, when the journalist began interviewing Nuon Chea, and his arrest in 2007.
Cayley spoke of mass killings of Vietnamese and Cham Muslims, saying regime leaders had a "systematic plan of genocide" for them.
The defendants knew about and participated in five main accusations against them, Cayley said, including forced evacuations into rural areas, enslavement of the population in labour camps and the widespread practice of forced marriages.
Missing from the courtroom is the fourth accused, Ieng Thirith -the regime's "First Lady" and the only female leader to be charged by the court - after she was ruled unfit for trial last week because she has dementia.
Led by "Brother Number One" Pol Pot, who died in 1998, the Khmer Rouge emptied cities, abolished money and religion and wiped out nearly a quarter of Cambodia's population in a bid to create an agrarian utopia.
Nuon Chea on Tuesday dismissed the allegations against him in a lengthy speech to the court that at times slipped into a history of the communist party.
"Whatever has been indicated in the opening statements is not true. My position in the revolution was to serve the interest of the nation and the people," the 85-year-old said as he read from a prepared statement.