Albert Einstein: "The supreme goal of all theory is to make the irreducible basic element as simple and as few as possible without having to surrender the adequate representation of a datum of experience." 

Hitler's explanation of the Holocaust is the best one:

Nature if cruel. Therefore, we too, may be cruel. If I don’t mind sending the pick of the German people into the hell of war without regret for the shedding of valuable German blood, then I have naturally the right to destroy millions of men of inferior races who increase like vermin. (Meltzer, 1976).

If Hitler had no compunctions about killing his own soldiers, why should he not also have the right to kill Jews.