During Communist revolution in the 1970′s, Cambodia engaged in a mass murder of anybody who they considered a counter-revolutionary. This included government employees, intellectuals, and anybody who displayed disapproval of the new regime as well as their families. The death toll was over 2 million.
A schoolhouse (top two photos) was converted into a processing centre where counter-revolutionaries were documented by having their photo taken with a number around their neck. They were then tortured into confessions. After this, they were taken out to fields blindfolded with their hands tied. Bullets were considered expensive and that the noise would draw attention. So the people being killed were usually clubbed with gardening tools and pushed into mass graves and frequently buried still somewhat alive.
The bottom two photos show headshots, but unlike the photo above, these people do not have prisoner numbers around their necks. These people are the killers, not the prisoners. Although most of them look very young, they are ones who carried out these killings.