The Spanish conquistadors were fairly brutal, but they probably did not want to exterminate the population of the entire New World outright through introduction of dangerous diseases. If only because they needed some slaves and servants for their luxurious lifestyle, plus women for their physical needs.
But in the 1500s, our understanding of contagious diseases was basically nonexistent and the first rational means of combating them (such as quarantine) were considered on par with prayers, holy processions and a judicial search for witches that might have caused the latest epidemic through deals with the Devil.
We might be doing something similar again, and again out of ignorance.
Wikipeda says that quarantine in medieval Europe had begun in the 1300s and gained it's name in Italy in about 1448. There's no evidence that it was seen as religious or mysterious, even if people then did not have a germ theory of disease.
The real issue is the people in the late 1400s and early 1500s did not appear to have any conception of widespread, genetic and experiential immunity to diseases spread directly between humans. The diseases that killed so much of the the indigenous population of the Americas really didn't have much impact on Europeans (for reasons that appear to combine both genetic ancestry and living conditions that had help force immunity long before). So the early missionaries and explorers likely did not really have a clear notion that they could be bring death to tens of millions of people, because that was basically impossible in their homelands.
"There's no evidence that it was seen as religious or mysterious"
That is not what I meant. I meant that for combatting diseases, religion and magic was used as often as things like quarantine and few people would try to hash out what was more efficient and what less.
This seems like a large-scale trolley problem. Colonization of the Americas represents throwing the lever from one track/timeline to another. Both timelines involve massive numbers of deaths that would occur earlier than on the other one. Both involve massive numbers of specific lives that existed which wouldn’t otherwise have.
It’s impossible to say, but it seems likely that the colonial efforts of the Britain, Dutch, Spanish, and Italians would have been directed elsewhere rather than vanishing, resulting in deaths over there instead of over here. I have to think that everything that was first invented/discovered in the Americas would have eventually been invented/discovered elsewhere, but for whatever the delay in time was, people would have suffered/died in some cases. Later, possibly a lot more European Jews in the 40s. It’s an alternative timeline with a corresponding alternative set of deaths.
Yes, if the science says so. Yes, but the question is: Do we want to do stuff like that again?
Human impact of American colonialization: 56 million deaths.