The Khmer Rouge were the communists in Cambodia. One of their notable leaders is Pol Pot, who was a vicious dictator.

They forcefully relocated cambodians, did forced labor, took all private property, etc. They usually did this by going into towns, and saying that they had to relocate people outside of the town, to protect them from bombings and such, but that they would be able to return shortly afterwards. Anyone who refused typically were killed. All these people were forced on long death marches into labor farms, where they were supposed to become farmers. Problem is, when you shove city folk onto a farm they often have no clue what to do, and can't grow shit. Anyone picking wild fruit or other means of scavenging were killed. Commercial fishing was outlawed. 12 hour work shifts became the norm, as many starved to death. Many of the people who struggled, due to lack of agrarian knowledge, were labeled as saboteurs and killed.

Books were burned, and people considered to be intellectuals, or related to education, were killed. After a while, even simply having something like glasses would get you labeled as a intellectual and killed. Even things like hospitals were closed. Bankers, and many business professionals were killed. Anyone that pursued artistic endeavors were killed.

Religion was banned, and anyone seen partaking in religious practices were killed. Muslims were forced to eat pork, and those who refused were killed. Priests and monks were killed. People who were ethnically not camodian were killed.

See the theme here? Basically if you weren't a skilled communist farmer, you were killed. The estimates of the death toll range from 1.5 million up to 3 million (20-40% of the population). They've found over 20,000 mass graves. The term "Killing Fields" refers to how they'd take large groups of people out to a field for their execution. To save ammunition, most of the executions were done using sharpened bamboo spears, spades, or pickaxes.

They also had the habit of taking photos of the executed. When they were forced out, entire rooms were found that were completely filled of nothing but photos of people who were killed.

The Khmer Rogue were only in power for 4 years. Pol Pot feared a Vietnamese attack, and decided to preemptively attack the Vietnamese. This fared poorly for them. Many defected from the Khmer Rouge, and some major people were captured. Those who remained fled, and started doing more guerilla style warfare, with camps that were supported by diamond smuggling, and support from the Chinese. This led to a sort of civil war, with the new government that was propped up by the Vietnamese going against the KR. Peace and disarmament treaties were often violated by the KR. In the '90s, the remainder of the KR started collapsing. Mass defections occurred, and infighting began (especially after some of the Cambodian royalty tried to win over a handful of the KR leadership, and refusing to deal with Pol Pot and others). This lead to Pol Pot being arrested and attempts for a trial of which he died shortly there after. He committed suicide after finding out he was to be handed over to a international war tribunal. Most of the remaining leadership surrendered afterwards, or were captured.

Here's a post I made else where on Cambodia that may provide some insight.

Pol Pot's revolution was an entirely peasant based uprising. Democratic Kampuchea was an attempt to turn the entirety of Cambodia into a peasant based agrarian society. The Cambodian peasantry didn't know private property. All land and property belonged to the French, the Aristocracy, and the Japanese. The Cambodian peasantry didn't know education. The French purposefully kept literacy and education rates as low as they could to prevent political unrest. Once the Japanese were run out of Cambodia, a new Aristocracy took control and through Prince Shinouk's attempts at neutrality Cambodia began to be infiltrated with Vietnamese. Cambodian peasant relations with the Vietnamese had always been sour as the Vietnamese were better educated and enjoyed higher privileges in Cambodian society than even Cambodian peasants. Eventually a military dictatorship came to power, one that allied with the United States and was complacent in the death of hundreds of thousands of Cambodian innocents. The peasants who had never earned much money were packed into cities to escape bombing runs, left with no money to buy food or provisions, left to starve.

According to the Khmer itself, the US bombing raid was the final rallying cry for the Cambodian peasantry, which fully allied itself with the Angkar, the Khmer Rouge before they fully adopted Marxist/Communist posturing. The Khmer state reflected what the Cambodian peasantry knew: It was an agricultural state, one that favored agricultural labor over education, one that favored full communalization of property, one that favored Cambodian ethnicity, one that favored traditional Cambodian peasant life. Property, education, money, multi culturalism were all symptoms of things that had kept the Cambodian peasants poor and destitute, and in the new Cambodian state, those traits would be eradicated.

Pol Pot was overwhelmingly popular with the Cambodian peasantry. Enough to empty entire cities through force. You know what happened to the Cambodian people afterwards and Pol Pot eventually began to alienate the peasantry through over work and dehumanization and using starvation as a weapon and means of control ("If someone is very hungry, the Angkar will take him where he will be stuffed with food."). When the Khmer Rouge were removed from power through collaborators with the Vietnamese a sizable portion of the Khmer Rouge forces went with Pol Pot into exile. These forces were the ones that came from the provinces within Cambodia that saw the least amount of strife, as certain districts did not suffer nearly as immensely as others. They were also propped up with US and UK funding but their entire goal was to remove Vietnam from Cambodian soil. In interviews post downfall Pol Pot was steadfast in believing that what he was doing was right, and he still had support from a faction of the peasantry. While I've never heard that particular saying in your question, Pol Pot's isolation from the absolute worst situations in the Khmer Rouge, his belief that all that oppose the Angkar were not true Cambodian peasants, and his continued support in exile gave him all the vindication that he needed. Sources:

The Khmer Rouge: Ideology, Militarism, and the Revolution That Consumed a Generation

Pol Pot's Little Red Book

Brother Number One: A Political Biography Of Pol Pot

Pol Pot Plans the Future

Nate Thayer's AMA and the reference interview as well.