I subscribe to r/catholicism, sometimes to read neat discussions from my co-religionists and sometimes^mostly to read awful ones. The basis of the original discussion is debating exactly how Satanic Islam actually is, so guess which kind this is. But that's probably more badreligion and I'm just looking at history, so let's start off with a bit about European Catholics:

Except for when they conquer natives, burn heretics etc, right?

With the response:

Oh please, not this whiny shit. The Aztec Empire deserved everything it got, and their neighbors fought with the Spanish for a reason. If it weren't for "colonialism", 20,000 hearts a year would still be ripped out on the altar at Tenochtitlan. Furthermore, unlike their "enlightened" WASP neighbors, the French and Spanish were much less racist and abolished slavery, complete with a papal excommunication of all involved with the slave trade. The execution of heretics was not the desperate flailing of an institution with something to hide, it was the suppression of social revolutionary movements that tore societies apart. The Albigensians and Hussites weren't innocent victims being crusaded for no other reason than unorthodoxy. Learn some history.

Maybe I just have terrible reading comprehension, but I interpreted the juxtaposition of the first statements to mean that other native city-states fought alongside the Spanish against Moctezuma due to the "reason" of being horrified with human sacrifice. I was always under the impression that the main motivating factor for the city-states was their own particular political goals and quarrels with other cities. For example, in two fantastic posts both u/Ahuatl and u/anthropology_nerd explain how the city of Tlaxcala used the Spanish to help in their conflicts with Cholula and eventually Tenochtitlan, and apparently Texcoco turned against its ally Tenochtitlan due to Moctezuma's interference in a succession crisis (this is also mentioned in Richard Townsend's The Aztecs). The original argument seems especially strange because the Nahua were far from the only group in the area to use human sacrifice. Kevin Terracino mentions Mixtec human sacrifice in The Mixtecs of Colonial Oaxaca, Bernardino Verástique describes Purhepecha human sacrifice in Michoacán and Eden, and here is a paper that talks about sacrifice of captives in Zapotec cultures. Like I said I might just be missing something, but I don't really see what human sacrifice has to do with various indigenous polities fighting Tenochtitlan, or what any of that has to do with the judgment that the "Aztec Empire deserved everything it got".

For the "20,000 hearts a year", u/Ahuatl also made a wonderful series of comments a few months ago going through the math of Aztec human sacrifice and the discrepancy between the archaeological evidence and the written accounts, where among other things it's mentioned that even 20,000 a year might be rather high. To be really pedantic, I could also mention that the 20,000 a year estimate was for the entire empire, and that Tenochtitlan had multiple temples with multiple altars, so even the singular "20,000 hearts a year on the altar at Tenochtitlan" isn't quite accurate.

For the "much less racist" bit, I don't know how you'd go about comparing the racism of European empires, but I'm not really sure that the Spain that created the casta system to judge people based on their color and ancestry and the France that kicked the Caribs off their own islands really deserve that much credit. In addition, while Spain at least did abolish slavery for native Americans in the 16th century, there was still a lot draft labor, so again, meh? Also, I like that we're just not even mentioning Portugal and the bandeirantes who openly captured Native Americans as slaves. Or that apparently Aztec human sacrifice is justification for all colonialism ever. And that we're ignoring the imperialism of Catholic powers in the nineteenth and twentieth centuries in Africa, Asia, and Oceania. Or that apparently Africans don't count as real people, or else I have no idea how anyone could talk about France and Spain ending slavery and the slave trade.

Unfortunately I know basically nothing about Albigensians, Hussites, and the treatment of heretics, so I'm not going to pretend to offer any historical critique.

Moving on, further down thread we have this map which baffles me. What is the "Muslim Conquest"? Are they talking about the Rashidun and Umayyad conquests of the 7th and 8th centuries? Because then why are there so many dots around Hungary and Austria and the Balkans - those seem more Ottoman-y. How is the map-maker defining battles such that there are there so many dots around Europe and Palestine, but virtually nothing in North Africa, the Middle East, and Central Asia? According to this map they literally just walked into Afghanistan and faced no resistance whatsoever (they didn't). Are we just deciding that any skirmish by any king or bandit who happened to be Muslim now counts as part of a "Muslim Conquest"? Why are we collapsing history and motives all together for Muslims, but then restricting the Christian side to Crusades? And why are we leaving out so much of the Crusades - like the First Crusade battles when King Colman of Hungary fought the armies of Emich and Gottschalk because he didn't want them to enter his country and massacre Jewish people, or the Second Crusade and the Conquest of Lisbon, or the Third Crusade and the Conquest of Cyprus, or the Fourth Crusade and everything about it, or the Crusades against Egypt getting collapsed into one dot even though they were fought in different places, or the Crusades against Tunis, or the Northern Crusades, or the Reconquista, or that one random crusade against Aragon. I mean, yeah, it's true, there won't be a lot of battles if you just choose to ignore like 90% of them.

Here are some other relevant sources that I didn't mention above:

Imagining Identity in New Spain by Magali Carrera about the casta paintings and race in Spanish America

City Indians in Spain's American Empire by Velasco Murillo et al. talks about the repartimiento draft labor system in various places

The First Crusade by Steven Runciman for the bit about King Colman of Hungary

The Northern Crusades by Eric Christiansen about the Northern Crusades and various other vaguely-religious expeditions in North-East Europe

Disclaimer: I am not a historian, so if I misinterpreted the original comments or wrote some bad history of my own, then please be brutal!