"I mean, the United States practically murdered an entire continent of civilizations and cultures"

You realize that this is largely propaganda and doesn't honestly or accurately describe the actual history right? If a history teacher taught you this, you deserve to get a refund. The actual history is a lot more complex, subtle, nuanced, and driven by biology and trade more than warfare. Most of the deaths were caused by disease and trade drove the warfare in most cases. Also, the warfare was rarely strictly along ethic lines but it was at times.

Perhaps you are thinking of the Spanish or something...