Think about it, before Europeans arrived on the North American continent; the indigenous population numbered somewhere in the neighborhood of 15 million souls.
When Christopher Columbus “discovered” the Americas in 1492, on his quest for gold and silver, the Native population, which he erroneously called Indians, numbered an estimated 15 million who lived north of current day Mexico. It was, by all considerations, a thriving civilization. Three hundred and fifty years later, the Native American population north of Mexico would be reduced to less than a million. This genocide was brought upon the Natives by systematic mass murder and also by disease, notably smallpox, spread by the European colonists.
This is why Thanksgiving can never really be free of the stains of genocide.