I understand that the Vietnamese didn't like them much, but what about other countries like China, North Korea, The USSR, and others? Were they in favor of the Khmer Rouge or was there opposition?
The USSR backed Vietnam. China (and implicitly, their erstwhile ally the United States) backed the Khmer Rouge, and continued to recognize Democratic Kampuchea as the official government for some time. North Korea generally didn't involve itself in Sino-Soviet pissing contests when it could get away with it.