Doomsday

“Consider,” said the Doctor.  “Every human is in competition with every other human, simply by virtue of living.  Each set of genes exists in order to propagate itself and none other.  Romantic partners, siblings, even children are in competition with you.”  He paused, stroking his beard.  “Cloning frees us from the necessity of cooperation; each gene-pattern is its own potential founder now.  It follows that every time someone else uses a resource, they are damaging you and your genes' ability to reproduce most efficiently.”

 

He pulled the lever, and the red numbers began the countdown.  “This, therefore, is simply self-defense.”