(Racism comes in so many varied and often subtle forms that it complicates talking about it; you need to be pretty specific about just what element/manifestation of racism you're talking about. In this case I'll discuss one of the uglier and most visible kinds.)
To my mind the worst form of racism is the belief that one race is superior to another and that the superior race should take action (legal, social, religious, etc.) against the inferior race as a result of that conviction. Examples are legion, and included nations as well as social/political organizations (South Africa (during Apartheid), Germany (during the Nazi reign), U.S.A. (during antebellum and Jim Crow), as well as the KKK, neonazis, etc.)
What boggles my mind is the idiocy and dishonesty of their position.
When I was 14 I was at a small summer camp in the mountains of North Carolina. Everyone working at the camp and everyone attending the camp was lily white; no one of African, Hispanic, or Asian descent. Almost everyone at the summer camp was from the surrounding Southern states. Racism was rampant but fortunately ugly comments rarely came up since there was not but a pale face to be seen or insulted. But I remember this one time the topic came up, and this one boy, whose name was the name of a kind of monkey, suddenly announced, "I am smarter than any black person on the face of the Earth." What made this seem instantly laughable, distracting me temporarily from the horrendous and vulgar racism, was that he was truly one of the stupidest people (of any genetic background) I'd ever encountered; he was almost sitcom human stupid or perhaps penniless Trump stupid. But he was serious, horribly serious, and I feared and felt for anyone of any non-white hue who might ever cross his path. Several of us there argued with him, tried to point out the ridiculousness of his statement, but he was having none of it. He was so unaware of his own place in the universe that one could hardly hope to convince him of anyone else's place within it. I was still young, but I had certainly come across quite a few people, made from all sorts of different genes, who I knew or strongly suspected were vastly smarter than I was. How could this idiot imagine he was smarter than ~1 billion or so (depending on what you count) people he'd never met? That is idiotic and worse deeply dishonest, as he lived in a major city in the South, and surely had encountered many black people who were infinitely smarter than he was, and he simply chose not to notice or believe it.
But it is not hard to imagine why he (and others) seem to need to believe such an absurd position, need to believe that they are better than an entire race, because things get rather confusing for them if they don't. The wildly racist often profess strong values, and no doubt most try to live in accordance with them. And many of their values involve a love of freedom, family, righteousness, justice, etc. So to avoid a deep and disturbing sense of hypocrisy they must write off the members of the group against which they stand. If all blacks people are inferior to white people then they can see a logic in allowing them fewer rights, deeming them unworthy of a full and equal position in the world, and any white actions taken against blacks are reasonable in defense of white interests. If these racists allow that some percentage of black people are actually their (individual) superior (across many facets of being, including intellectually), then what are those racists going to feel? I think some parts of their brain would cry foul, detect their hypocrisy, detect their injustice, and threaten their ordered world view. And so it is only a stable and comfortable position if every white man is inherently superior to every single black man (a position requiring ample employ of idiocy and dishonesty).
I keep wondering if that particular camper ever wised up. I hope he has.