To look each other in their European eyes

The reason the Boers of South Africa became the perpetrators of a vicious slave-holding culture was because they never formed a civil society amongst themselves. They lived far apart from each other on giant ranches whose land could barely maintain their livelihood. They managed to justify to themselves the (mis)treatment of Blacks like chattel because they never actually had to look each other in their European eyes and engage with each other in the ordinary activities of mutual aid. 

The terrain and the society that grew out of that ecology dehumanized them, and so they treated others inhumanely. Not that the Blacks of the region were all that much better. 

"King Tchaka, who at the beginning of the nineteenth century united the Zulu tribes in an extraordinary disciplined and warlike organization, established neither a people nor a nation of Zulus. He only succeeded in exterminating more than one million members of weaker tribes." (The Origins of Totalitarianism, Hannah Arendt, 1994; page 192).