>>17923194
There were pro-Vietnamese members (at least relatively speaking) of the Khmer Rouge and they were mercilessly eliminated by Pol Pot's troops. There were more of these people in the eastern zones, apparently. The Khmer Rouge came to power in 1975 and really escalated to mass killing of anyone Vietnamese, pro-Vietnamese, people suspected of being pro-Vietnamese, in 1978. This is from a book by Michael Vickery.

The east was the zone that had more of the "old" communists who had cooperated with the Vietnamese in the 1940s and 1950s. They didn't always get along with the Vietnamese, but were considered more "liberal" as well, more relaxed anyways. Different ideas. Probably more similar to the Vietnamese communists. They also wore fatigues and mixed uniforms rather than those black uniforms.