" It’s harder to kill people. The empathy is so much stronger that the mind must invent new reasons. But, if we can somehow link it to our own survival, the mind will make the devious twists and turns necessary to rationalize it. We’re very good at that. But it changes people. They learn to hate. Your wolf doesn’t need to hate what he kills. It would be easier if we could kill without compunction, like your wolf does, but then, we wouldn’t be human. "
― Jean M. Auel , The Shelters of Stone (Earth's Children #5)