This isn't really true, many conquerors did not engage in settler colonialism and more often than not they often just conquered a people and extracted taxes. That can be done in a very evil, exploitative way but the point is that wiping out the local population is far from the norm, most of the time they're just forced to pay tribute and allowed to continue their existence as usual.
Consider for instance the Palestinians. The laymen thinks of them as descendants of the Arabs who conquered the region and displaced the local population just like the Zionists did 1300 years later but its not true, genetic studies show that they have lots of overlap with modern Jews behind only the local Druze and Bedouin. That's the pattern in most places; outside group conquers the local folks, extract taxes and tribute from the farmers and elite, and over centuries there might be cultural assimilation.
Only if they're white like the Boers.
So who is being referred to here when they speak of "Boers" and why is it okay to talk about killing them?