There are people in pre-modern times that saw the horror of what was happening and did their best to fight against it. There were ex-settlers that chose to join the Native side of the struggle because they knew settlement and genocide was wrong, even if they didn't really have the language for it yet. Hitler was inspired by the pre-WW2 actions in the Americas against the Native population.

Though I won't argue that people today have even more benefit of hindsight - especially because we can see it happen live on the internet!