Can it be considered dark, if we're thankful?

It's based on a myth of settlers and natives getting along when in fact the US was engaged in a campaign of deliberate genocide. Thanksgiving is about pretending something nice happened when it's one of the greatest crimes against humanity to take place on American soil.

Yes and holidays can be very dark for a lot of people because of things that happened in the past.

pillaging the natives? idk

Probably the genocide

Invading this land. stealing it. then Slaughtering the natives. Then "celebrating" this glorious invasion and conquering ways. Doesn't get much darker than that.

Stealing everything from the Natives

Found a fitting PDF for this.

http://seedofabraham.net/Thanksgiving-Day-Pagan.pdf

This was the one of the last times the colonists got along with the wompanoags before they murdered them all

no secret - - goes like this - us crazy white folks slaughtered all these indians....so in order to make it a party everyone got together for dinner to talk about what a WONDERFUL time everyone was havin' with the slaughter thing.....was SO much fun they decided to make it a tradition....
But they eventually ran outta indians to slaughter - so NOW we slaughter a bunch of turkeys & eat 'em instead....