HistoryPorn

Because it's a defense argument.

Defending the weak or defending oneself are the only legitimate causes for violence in most Western societies. Even the Nazis framed much of their violence as defense of the race (and by extension, humanity) against Jews and bolshevists. Hardly any group says downright "fuck those guys, because I really don't like them", is almost always a threat. Immigrants take our jobs, welfare state or women, gays make our poor little kids gay, etc. etc. So you never hate a human, you hate a threat agent.