Reading is a technology for perspective-taking. When someone else's thoughts are in your head, you are observing the world from that person's vantage point.
Technology, ideology, and social and cultural changes periodically throw out new forms of violence for humanity to contend with.
Fiction is empathy technology.
As technology accumulates and people in more parts of the planet become interdependent, the hatred between them tends to decrease, for the simple reason that you can't kill someone and trade with him too.