you don't need to worry about millions of people being killed, when they are vermin instead. human life is sacred, but nobody cares about vermin, right? in fact they are a plague that must be dealt with so humans can thrive.
they can easily demonize an immutable characteristic some people are insecure about and turn that group into the enemy, rinse repeat.
When being "desperate to look cool to everyone else" overrides common sense and your humanity, you get this. "Those who can be led to believe in absurdities can also be led to commit atrocities." - Voltaire (The Nazis of WWII did this very well. The "Pro-Palestinians" are proving it, again. Those who don't know history are always doomed to repeat it.)