It's called fear.
All those with power fear losing power.
Black people have been treated so bad they fear what will happen if blacks break free from oppression, they fear the retaliation they deserve, yet they cannot fathom the fact black people don't even want vengeance just freedom because they can only think with their own logic.
This is why so much energy and resources have been dedicated to crippling the black community for hundreds of years. This why J Edgar Hover who has buildings named after him and is considered a hero in this country stated the rise of a black messiah must never be allowed to happen.
So if you are black and proud you in direct defiance of America by default.
This is why they call the black panthers a hate group, being pro black, aiding the black community, doing anything positive for black people is an aggression act towards whites and towards America because it's not in their interest of blacks to be self sufficient and powerful.
That's the macrocosm. The microcosm, being the dumb ass bum ass everyday cacs believe they are superior by default. This is what they are taught and what is reinforced to them in society. So when a black person is buying into that and contradicts their reality they become enraged. White supremacy is a religion, they get mad the same reason muslims get mad at images of the prophet. It is blasphemous to them for a black man to be proud and confident, and they are threatened by that energy. They respond in that fashion because something in their DNA tells them to be afraid. A strong willed black men is a threat to everything they know and hold as true, so they want to diminish that by any means.