While you're right in pointing out that what people do when angry is most often closest to their "true colors", it's also undeniable that many racists happily assume the angry state of "undesirables" is their "true" form... because they're racist assholes and cannot imagine certain other races/minorities as normally nice/kind.

Probably because they antagonize them and then go, "see you are capable of being angry, and I only see you all being angry (because I'm an antagonist) so that justifies my ignorant, self-produced belief!"