May a nation of a million beings destroy a creature who otherwise will infect all with a fatal disease? Yes, you will say. Once more: ten starving beasts hunt you, that they may eat. Will you kill them to save your life? Yes, you will say again, though here you destroy more than you save. Once more: a man inhabits a hut in a lonely valley. A hundred spaceships descend from the sky and attempt to destroy him. May he destroy these ships in self-defense, even though he is one and they are a hundred thousand? Perhaps you say yes. What then if a whole world, a whole race of beings, pits itself against this single man? May he kill all? What if the attackers are as human as himself? What if he were the creature of the first instance, who otherwise will infect a world with disease? You see, there is no area where a simple touchstone avails. We have searched and found none. Hence, at the risk of sinning against Survival, we — I at least; I can only speak for myself — have chosen a morality which at least allows me calm. I kill — nothing. I destroy — nothing. (ch. 2)
Jack Vance, The Last Castle, Spatterlight, 2012 , Kindle Edition.