Also, China was an ally. Some of the worst stuff happened under the old Republic - they pretty much had a policy of genocide against Tibetans in Sichuan. I don't remember if it was actually the KMT (or some of the warlords who allied them), but they were quite brutal. Then the KMT fled to Taiwan, and the CCP became the "bad" guys.

While the CCP has a mixed history, they do relax the One Child Policy for ethnic minorities (I think Tibetans are allowed 2 children), so it's hard to suggest they are actually genocidal. Nationalistic, yes, but that nationalism recognises the rights of minorities (as long as they don't try to rebel). They did a lot of dodgy stuff in the Cultural Revolution, but it's not like they are given a free pass on that.

I'm not sure about the Great Leap Forward, but that was more an outrageously stupid policy decision than a deliberate crime. It only lasted 3 years. While Mao should have ended it sooner, it's not like he kept it going for 10 years, like the Cultural Revolution. Perhaps it really took 3 years to weed through the false reports of progress, and to realise it wasn't just growing pains, sabotage, or natural disasters. Besides, it wasn't really targeted at any ethnic group - all rural Chinese were hit pretty hard.

No, it's actually simpler.

First, Germany invaded other countries. They had to fight it back, else there would be a huge Nazi power state occupying all of Europe which would be a big thread (and contender for the same resources).

Second, the German "eugenics" program was not birth control or DNA sequencing but killing people they deemed invalid, inferior etc. By the millions. Not even comparable.

Third, China is not a deluded small-ish country in Europe, thinking it can take over the world by itself. It's a huge behemoth superpower.

They would die too. They need us as much as we need them. Employment is a huge issue in China. The unemployment that would result from the loss of US demand would cause a breakdown of social order.