This topic is far more important than many will give it credit for being.

To the extent that we talk about white Europeans' (frequently, perhaps mostly) negative impact on other cultures, a valid starting point would be an effort toward understanding both migration and conquest as two primary dynamics that shaped the ancient world.

For instance, it's easy to see the harsh, brutal impact of the Spanish on Meso-, Central-, and South American peoples. The forced conversions, the religious torments, the genocidal effect of disease all make it unavoidable. What is much more difficult, however, is to talk about the cultures that were largely eliminated by the genocidal effects, for instance, of Gaius Iulius Caesar's incursions into Gaul and Brittania. As someone of Brittanic/Scots/Irish stock, I can say with certainty that I have no real idea of any cultural identity that was not imprinted on my ancient forebears by virtue of Roman conquest. The religion of my ancient ancestors was largely wiped out wholesale by the Romans, and any remnants were definitely ground into the dirt by Holy Mother Church. The same applies to the relevant cultures.

It's no surprise, then, that a culture that has been scrubbed clean of identity finds it easy to do the same to the next group down the line in the next migration. I'm thinking of my multi-great grandfather who had no misgivings at all about shooting the Indians whose land he had stolen here in what is now West Virginia.