He may have been referring to the notion of the "White Man's Burden" -- the imperial doctrine that Europeans had the right and the duty to rule assumedly 'inferior' peoples. One exploration of the way that this "civilizing mission" and imperial genocides go together is Sven Lindqvist's Exterminate the Brutes, but for overall treatments of the Herero Genocide in particular I would recommend Casper Erichsen and David Olusoga, The Kaiser's Holocaust. To look at it in retrospective, see Volker Langhben and Mohammad Salama, eds, *German Colonialism: Race, the Holocaust, and Postwar Germany.

In the meantime, I'll leave you with some direct quotes:

General Von Trotha, Speech to German troops, October 2, 1904

The Herero people must ... leave the land. If the populace does not do this I will force them with the Groot Rohr [cannon]. Within the German borders every Herero, with or without a gun, with or without cattle, will be shot. I will no longer accept women and children, I will drive them back to their people or I will let them be shot at. These are my words to the Herero people.
 [Signed] The great General of the mighty German Kaiser

Missionary Elger, on the Herero Massacre in German South West Africa, Karibib, 1906

And then the scattered Herero returned from the Sandfeld. Everywhere they popped up —not in their original areas - to submit themselves as prisoners. What did the wretched people look like?! Some of them had been starved to skeletons with hollow eyes, powerless and hopeless, afflicted by serious diseases, particularly with dysentery. In the settlements they were placed in big kraals, and there they lay, without blankets and some without clothing, in the tropical ram on the marshlike ground. Here death reaped a harvest! Those who had some semblance of energy naturally had to work— It was a terrible misery with the people; they died in droves. Once 24 came together, some of them carried. In the next hour one died, in the evening the second, in the first week a total of ten - all [lost] to dysentery - the people had lost all their energy and all their will to live