I think that there is an argument here. You will need to provide context for the situation in Europe and the situation that native Americans had. Was their culture always the pristine, perfect society that it is often portrayed as? Was the Aztec culture a positive force for human rights?
Your argument will have to justify the obvious tragedies of the conquest by pointing to the overall good to humanity that American society eventually resulted in. And to do this:
1. Contrast the Religious Monarchies of Europe with the Free Republic of America.
How does concern the Native Americans? Whatever was happening in Europe, however bad it was, had no impact on how the Native Americans were living. Are you saying that the impact of the Free republic of America is better than the negative aspects that the European monarchies had on the Native Americans? One could make the argument that the if the European didn't come to America in the first place, the Native Americans wouldn't have had to deal with the Religious European monarchies.
2. Contrast the tribal, warring, and primitive societies of Native Americans with the modern American society.
and then show that the result of the conquest is better for humanity than either.
As a matter of fact, I think that this is a winning argument that I agree with. Europe was a travesty, led by egotistical, inbred, monarchs all squabbling over world domination. And the pristine image of early native Americans ignores the masochistic, primitive, uneducated cultures that they had.
Tribal warring, human rights depravation and other practices of the Aztecs, etc that you mentioned can not be used to characterize the entire Native Americans. Yes, they were practiced and common among certain tribes, but how can you say that those practices are any worse than the wars or other tragedies of the Free American Society?
A question someone can ask is "The Native Americans lived in their culture (how ever primitive it may seem to us mondern folks) for thousands of years. How can you justify that their culture or life style was worse than a 500 year culture that has already experienced tremendous overpopulation, wars with millions of casualties, environmental damages?"
Yes we brought them technology, idea of human right, and other BS of the New World. But honestly, do these "positives" outweigh the past tragedies that the Native Americans had to go through?
In my mind, no.
I guess this is what you call "respecitvely disagreeing"