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They said communists didn't commit a genocide, not that the US didn't.
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What the US colonies did to natives could reasonably be described as a genocide, but it was very slow, and often fueled by apathy and self-centered, quiet bigotry. I think I read that 2/3rds of Native Americans were killed unintentionally by diseases that colonists brought over, right? Then the last third were killed off or moved slowly over hundreds of years, kind of like a death by a thousand cuts. Not even in the same league as the Holocaust. That was very intentional, organized, targeted, and fast.
However, to your point, the fact that Americans deny that a genocide happened here is very problematic. Maybe not as problematic as Holocaust denial, but not every atrocity has to be a competition. It was an awful, horrible thing that our ancestors did and we should acknowledge it. Pointing at something worse that also happened doesn't make it better.
Attempting to subvert the thought experiment only makes things worse. The trolley is full of child prodigies, all future geniuses that will cure cancer and solve the world's problems. By sticking the lever halfway you kill all of them. The only way to save the child prodigies is to choose, left or right.