"What is it to live":

I always find all people, no matter how I look at them, with kindness or malice, taking care of one thing: How to serve the preservation of the species. And they do it, not out of love for the species, but because there is nothing older, stronger, uncontrollable, and more incomprehensible than this instinct within them, because it is true that this instinct, is literally the essence of our species, the essence of our herd.

Although we manage quite quickly, I can say, with our usual natural shortsightedness, to distinguish from a distance of five steps our fellow beings, into useful and useless, into good and bad people, however, if we sit down and evaluate and think about the general result of this distinction, we end up with a terrible distrust. The distinction does not satisfy us, we are not sure and in the end we give up. Ultimately, perhaps the most useless, the most harmful person may be the most useful for the preservation of the species. 

Because this person - the harmful one - preserves in himself or in other people, various instincts that without them humanity would have long ago become dull and corrupt.