I don't think they're evidence that the racist transatlantic slave trade was worse than the non-racist Roman world.

They aren't. However,

"...if a slave runs away into the forest in order to avoid work for a few weeks, upon his being captured his Achilles tendon is removed for the first offense, while for a second offense... his right leg is amputated in order to stop his running away; I myself was a witness to slaves being punished this way."

And similar punishments for marronage--from being castrated to being slowly roasted to death--are reported from different regions throughout the Americas.

grants context to your statement that "I never heard that the Confederacy lined their roads with crucified rebels."

we think that xenophobia is a built-in feature of the brain

Xenophobia and racism are different things.

atrocitologist

Is that ... a Culture ship name?