The planet is a living, evolving thing, and that includes the humans who live here.
Sure, get all the phony moral righteousness all rolled up into a big ball and try to use it to explain how the US treated the natives, but it doesn't apply to just us, because that's what humans have been doing to each other since our species became a species. It's nothing new to this continent, or this country.
What about the many more cultures and languages that have died out, mostly through being conquered by other cultures, that had nothing to do with US expansion, and manifest destiny? Yes, it would be nice to force, un-naturally, those cultures and languages to still exist. But it just doesn't work that way. At least with modern technology, we can preserve in digital form, any cultures / languages that are dying out today. It's better than nothing.
As for slavery, wasn't it the UK and the US who were the early, significant countries that banned slavery? If you want to make an argument about the horrors of slaver, go right ahead, because any form of human bondage is horrible. But don't lay the blame with the countries that actually took the first steps to abolish it.
btw, I don't ever see those on the left complaining about current slave trading. Hell, the Brandon administration let over 300K unaccompanied minors into the country, and didn't bother to track them. How many of those poor souls were forced into slavery, or to become a sex slave?
Where's the lefts phony moral outrage against that? Nowhere. When Trump had ICE raid an illegal weed farm here in Taxifornia, and freed some of those poor kids, all the left did was cry fascist for deporting illegals who engaged in slave trading. Think about that for a little while.
IF the left had any consistency as far as any of these issues go, I'd be on their side. But the sad reality is that they only use an issue when they think they can take down Trump. That's no way to behave, let alone behave politically.
So save the phony moral outrage. Nobody is fooled by it.