IMO the KR relationship with Vietnam proves that it was some kind insane death cult.
The second secret agenda item of the KR (after destroying Khmer society) was to retake Kampuchea Krom (Little Cambodia), which is that part of Vietnam from about Tay Ninh/Saigon all the way down to the the tip of the country at Ca Mau. The territory had been lost by Cambodia several hundred years before, and the KR believed it still belonged to Cambodia (hmmm.... sounds just like the Muslims of today).
Anyway after multiple midnight attacks on Vietnamese border villages - in one case wiping out an entire town of about 1,500 by hand and knife - the KR made the big mistake of launching a rocket attack on the border town of Chau Doc.
That was the final straw, and the Vietnamese turned on the rather hardened war machine one more time, taking Phnom Penh in less than one week.
Pol Pot and his cronies ran into the jungle and conducted a useless insurgent campaign for the next 10 years.
Cambodia was going to be the PROOF that a TRUE Communist society, REBORN in TRUE Communism would be a sucess. That is why ALL intellectuals or others with even a hint of education had to be eliminated.
That is also why NO western intellectuals CRITICIZED the KR when they took power, even though the westerners knew what was taking place in Cambodia. This was Communisms big chance to start with a clean slate and show what a true “workers paradise” Marxist society could be.
Cities were emptied to get the people back to the root of collectivism on farms. The only way to get rid of the seeds of capitalism was to wipe out and entire generation of those infected by capitalism. This was an expression of “The Cultural Revolution” in full bloom, with the blessing of Peking.
Seeing as this was a China inspired government, that was the REAL reason that Vietnam, as a client of the USSR, went on the attack. Russian communism was slightly more humane than Mao’s version of the filth of Marxist/Leninist Stalin-ism.