It’s generally not very hard to incite violence across groups in the Middle East, especially when you consider how bad the outcome might be for the losing side. Case in point, the Alawites who lost control of Syria and are now persecuted by the new government.
“ While most of the killings were carried out by IRGC and Basij forces, reports received by Iran International indicate that proxy forces from Iraq and Syria were also used in the crackdown. The deployment of non-local forces suggests a decision to expand repression capacity as quickly as possible.”
The notion of some well-defined "people" is a fiction that ruling powers use to keep humanity's innate tribalistic tendencies pointed outward at their adversaries.
The truth is that the powers-that-be consider themselves to be above "the people", and will dispose of you as soon as you become inconvenient.
Usually mercenaries mean people doing it for money not ideology who get paid significantly more than your average soldier.
The end of the regime was brought about by an incursion into the Vietnamese border town of Ba Chúc, resulting in the massacre of more than 3000 civilians. Vietnam invaded, toppled the Khmer Rouge and brought an end to the executions although civil war would continue for much of the next decade.
For these actions Vietnam was extensively sanctioned[1]. The parallels with ongoing conflicts today are hard to ignore.
[0] https://en.wikipedia.org/wiki/Khmer_Rouge#Crimes_against_hum...
[1] https://en.wikipedia.org/wiki/Cambodian%E2%80%93Vietnamese_W...