I think a really basic problem with how white people in the U.S. learn and understand history is that slavery is positioned as like "a thing that happened" and not an acute moral failing and profound tragedy on the scale of genocide.

A huge number of people -- on the order of millions -- died in the Middle Passage or once on land in the Americas, either during confinement or while being marched from place to place.

And, that doesn't even touch on the number of people who died due to abuse and neglect once they were sold. But, also, surely that sentence is enough: People were sold. People. Sold.