I believe the accepted narrative insofar as the New World is concerned is that it was largely disease that eliminated the population. There's a theory that is posited in Charles Mann's book 1492 which indicates a probable explanation for extremely high mortality rates due to susceptibility to the pathogens the Europeans introduced. The gist: natives of the New World had immune systems conditioned and selected for parasites, which appears to be an either/or proposition, which may explain, in part, the relatively disproportionate fatality rates among the native populations. Relative to that I think the warfare conducted by Europeans pales in comparison.

The best explanation I’m aware of for the disproportionate death rates by Europeans vs Native Americans by disease is mainly population density of European cities was dramatically higher, leading to a lot of diseases that would permanently spread throughout the population until the combination of evolution and immune systems brought it to a symbiotic, fairly neutral relationship.

Which didn’t remain so neutral when applied to fresh populations.

Native Americans simply weren’t playing host to as many diseases, and persisting virulent ones