The Khmer Rouge killed about 25% of the Cambodian people. They become aware that only an external force could change the power structure, and this happened with the Cambodian–Vietnamese War.
If they continued killing people, then less people would produce food (they used low tech for farming). Starvation would increase, and the cycle continue.
Is there a study about what would have happened if Khmer Rouge remained in power in Cambodia? Would the country have descended into chaos, and become a failed state?