18 million would be on the high end for the estimated indigenous population of pre-Columbian North America. 90% of those were wiped out by imported diseases before the United States even existed. It would be quite a trick to murder 100 million people based on a starting population of only 1.8 million (not all of whom, or even most of whom, were killed, by any means).

It is not reasonable to blame the United States for things that happened before it existed.

Also, it is pretty much accepted nowadays that killing off indigenous people is a Bad Thing. Unfortunately, the same is not true of communism. There are still plenty of people who think it's a wonderful idea.

Please don't treat divisive, inflammatory topics as mere ammunition for trivial internet arguments. That's a common pattern, and it makes discussion both uncivil and unsubstantive.