It's a psyche thing. Think of the Southern plutocrats, the slavers in the antebellum days. If they'd simply shut the fuck up about it, the rest of us probably would've let them keep their slaves, at least for a few more generations. But they insisted that we all validate their bullshit worldview that Black people weren't people. And they fucking attacked us over it. Reality only defends.
You can also say the same thing about WWII. The Nazis just couldn't tolerate the existence of neighbors who don't believe the same hateful bullshit they do.
I also see the US Civil War and WWII as the same kind of thinking that produces a mass shooter. It's people who chased their denial mechanism all the way to death. Rather than admit they are just full of shit, they lash out and try to take down the world.
Tangentially related: Christianity, to which both the Nazis and slavers subscribed, is a death cult. Never forget that. Once a death cult conquers its territory, it immediately schisms and attacks itself.