According to Wiki, Pol Pot killed 2.5 million out of a population of about 8 million.
FYI do you know how they did it? Killed more than a quarter of their own population? We have that movie, The Killing Fields, where masses of people are killed by suffocation, but that's drama. You can't really kill millions of people by putting plastic baggies on their heads. The Khmer Rouge was always a small minority and too much overt killing would have produced a rebellion.
The Khmer Rouge abolished money. The elimination of money caused a complete collapse in the social division of labor. Without pricing every sort of production became grossly inefficient. Agriculture was no exception, and most of the people who died simply died of starvation and starvation related diseases.