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.