We in the US have technically acted against native Americans (and don't get me wrong, I think we have nothing to apologize for -- it was war and just like others, the losers lost badly).
The European instinct? Nah. That was fomented by Tsarist Russia after they took over the eastern lands of the Polish-Lithuanian commonwealth with it's masses of Jewish people. The Germans under H took it up as a way to explain why they lost WWI when no battles were fought on German lands.
in many ways if the allies had fought straight through on to German lands in WWI, WWII may never have happened.
The Germans couldn't understand that their leaders knew they had lost the world and were on the brink of defeat even though German troops were on other country's lands. the leaders then to save their own skin encouraged the theory of a stab in the back and it was easy to frame the biggest outsiders left in Germany -- the Jews.
the other big outsiders -- the Poles -- already clawed back their own lands.
If we had fought on to German soil, their apetite for war would have gone away -- just as the French appetite waned (and we can't call them cowardly in WWII knowing how many of the blood-wrenching battles of WWI were fought on their soil) and they would have KNOWN they were defeated by their own faults. No one to blame and an acceptance. This would have saved millions of peoples lives.
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Ok, ok.. I know it is a big 'what if' scenario, but it's just my opinion
Even in the Indian wars the USA never had a long-term or consistent policy that could be called genocide. There were always those who wanted to absorb or assimilate the Indians instead. As you say, while terrible atrocities were committed, this was never official government policy, or at least not for long.
I find the famous Sand Creek Massacre enlightening. US troops, actually more sort of untrained and poorly officered militia during Civil War committed atrocities. (Most real officers were fighting back east.)
Even at this massacre, arguably the worst in US history, the atrocities committed by the US troops did not come close to those routinely, as a matter of course, committed by the Indians they were fighting. No mass rape, no dragging along of captives for long slow torture later.
Yet the very practices excused when committed routinely by Indians are somehow an indictment of America when committed occasionally by US troops.