A face we can never forget: Nuon Chea had a part to play in the death of more than 1.7 million people.

Beginning in 1975, the Khmer Rouge executed ethnic minorities, professionals, intellectuals, and almost anyone who had an education. They executed people who had connections to past governments, as well as Christians, Muslims and Buddhists. They executed artists and musicians, and many who lived in cities. They executed people who wore glasses.

Pol Pot, the leader of the Communist Party of Kampuchea (the Khmer Rouge served as their armed forces) known to history as a "genocidal tyrant" sought to create a "master race" through social engineering.

He idealised the rural, agricultural communities that worked on the land, and imagined a Cambodian utopia untouched by the evils of wealth and religion. Pot's vision was regressive, wanting to take the country back to a time before technology or development.

In Cambodia, renamed Kampuchea, 1975 became 'Year Zero'. History was erased, and the culture – as well as much of the population – was to start from scratch.

Chea was known during the regime as 'Brother Number Two', the right hand man and brother-in-law of Pot.

A site in the village of Choeung Ek has been converted into a Buddhist memorial for the thousands of victims murdered there. Dozens of mass graves are visible, and when rain falls on the grass, bones, teeth and clothing often begin to surface from beneath the ground.

Chea was also instrumental in setting up more than 150 prisons for political opponents. The vast majority of inmates were killed.

Two days before the Khmer Rouge fell in 1979, Chea himself ordered that the head of infamous prison S-21 kill all remaining prisoners. Those orders were carried out.

It wasn't until 2014, 35 years since the fall of the Khmer Rouge, that Nuon Chea received a life sentence for his crimes committed against humanity.

Judge Nil Nonn said the court found evidence of, "a widespread and systematic attack against the civilian population of Cambodia," and found Chea guilty of murder, extermination, political persecution and other inhuman acts related to the mass eviction of city-dwellers and executions of enemy soldiers.

In 2018, he was formally convicted of the crime of genocide.

At 93, significantly older than the life expectancy of the average Cambodian – which is just 68 – Chea died of natural causes.

His wife did not leave his bedside.

The victims of Chea were often tortured until their last breath and isolated from their families. Millions died decades too soon.

His death is a timely reminder of the dangers of racism and extremism.

As Youk Chhang, the head of the country's Khmer Rouge archive put it, "The crimes he has committed will always be a lesson for us all in the future."