The Chinese were definitely aware of it. But for the most part the Chinese in Cambodia are Chinese Cambodian, they were here before China turned communist. CCP is allied with Khmer Rouge, as the Khmer rouge both keeps vietnam in check and give them influence in the region. Vietnam and China have been historical enemies before they even turned communist.

Honestly, everyone kinda knew what Pol Pot was doing. They just downplayed it cause to acknowledge it would go against their political interests. Even the American and the UN downplayed it, since everyone was kinda making business with the Khmer Rouge. The Khmer rouge got their weapons from somewhere, and it wasnt from Vietnam. Vietnam only liberated Cambodia after leaving it to rot for years because they need a stable border, and historically have wanted control of the region.

Before french colonialism, Cambodia at its lowest was a vassal state split between Vietnam and Thailand. Part of the reason of them accepting French "protection" was because they were at high risk of loosing full sovereignty and divided completely between the two neighbors. It was a rock and a hard place situation. Cambodians modern alliance with China is for similar reason, yes there's corruption and people taking private interest in Chinese investment, but strategically it's to keep the neighbors at bay. There's still land & cultural dispute between the three nations. If they allied with US, who's also allied with Thailand and Vietnam, should the neighbor decide to challenge Cambodians sovereignty again, the US will not back Cambodia. Between the three, Cambodia won't be the important piece. Look at the map, China share a border with Thailand and Vietnam, in Cambodia, most Chinese businesses are at the border. There's a reason for that.

Again, like any corrupt country, there's always people who's benefiting from bribe from Chinese corporation. On the other hand, a lot of bridges and roads are being build by them, that may seem insignificant to some but having the country connected certainly help boost the economy. Most common people here aren't a big fan of Chinese businesses and people, they're rude and the business can be exploitative, but then it's not like they're the first foreign businesses that are rude and exlotitative. Some factions of the ruling governement also know that China can fuck them over when necessary too, but Cambodia do not have the capability of standing alone.

Essentially the issue is very nuanced, and I suggest not putting it in Binary of good vs. bad, or china vs, US. Small and weak nations do not have the luxury of making these decision on the basis of morality, they need to survive and they can be easily crushed if not careful. Cambodian tried to remain neutral during the Vietnam war and look where that got them. This country have been through about 4-5 wars, famines, and other disasters in the past decade. If it's one thing Cambodian are getting good at, it's making tough decisions with a lot of bad choices.

Every ethnic group, Khmer, Cham, Chinese, and hill tribes, they all went through their own special hell by the Khmer Rouge. The Khmer Rouge wanted to wipe every thing that makes everyone different, a literal blank slate. All culture, religion, history. The only thing they idolized is the "golden age" of Angkor. To be honest, though they say they're communist, their behavior is much closer to Facist, specifically like the Nazi idolization of the Roman empire, might makes right, the superiority of a group of people over others, the destruction of the intellectual community and so forth.

I'm Cambodian with historian Parents who grew up through out the entirety of Modern Khmer history straight from independence.