I don't know what this fallacy is called but the whole "if they didn't do it then someone else would" argument is often used to defend some terrible atrocities. I don't agree with it.
I get whereyour coming from- it is not, and cannot, be a defense or an excuse.
Before any ethical system, people and institutions are accountable for what they do, at least their reputations (for the dead) are accountable, based on the standards of their times or later.
At the same time, as a statement of probability the odds that someone within his century would have gotten around to employing similar criminal methods in the Americas, it is completely valid to say to macro picture of European treatment of, and impact on, Amerindian natives would have been the same as OTL.
righteousness have to be protected though. You need people willing to fight for it, not just remove specific bad people
Indeed, you would need a broad sea change in cultural practices and enforcement of the highest aspirational standards of the day to change results on the macro-scale. Morally it would have been worth it. It just wasn't likely.
Europeans already had a big difference between ethical standards expressed (at least aspirationally) and routine practices. Hypocrisy is quite old.
European wars, conquests, enslavement, expulsion, and deliberate property and resource destruction certain made a life and death difference numbering in the millions. Not the majority of millions lost probably. For that we can blame morally unaccountable microbes, and for a thinner slice of the pie of millions, morally unaccountable introduced invasive plant and animal species (various pigs and rats) disrupting ecology were the cause.
---To prevent or mitigate a major fraction of the epidemiological, ecological and megadeath disasters that befell the American natives from abrupt contact with Eurasians, one would need constant tending from ASB guardian angels. That's why Native American victory scenarios are so few, limited in scope, and strictly relative. But worth doing more of.