...even without the issue of the African slavery ( which was of an industrial-scale and brutality that makes it quite unlike most other historical slavery )...
If I am understanding you correctly, that you think American slavery was worse than any other slavery and that that is what makes it "unlike most other historical slavery", you need to go and brush up on your history. American slaves were treated like royalty compared to the largest slave nations throughout history. Here's an example: "Spanish historian Fray Diego de Durán reported that 80,400 men, women and children were sacrificed for the inauguration of the Templo Mayor under a previous Aztec emperor" (source: https://www.history.com/news/aztec-human-sacrifice-religion
). They cut the still-beating hearts out of the chests of men, women, AND children. I'm not saying slavery here in the US was a cake-walk; on the contrary, actually. But don't try and spout off that it stands alone as the most brutal slave nation in history.
No, I guess you did not understand me correctly. I did not say anything about American slave treatment. I said African slavery, by which I meant the entire sorry Atlantic slave trade, perpetrated over several hundred years, on an industrial scale, largely by the major European trading nations ( the Portugese, Spanish, Dutch, British and French Empires ), with the willing participation of some African leaders.
When the Europeans started trading extensively, and sending missionaries to West and central Africa, they found a patchwork of nations, many of which had already been influenced for centuries by the spread of Arab culture across Africa, including a slave trade, largely serviced by local wars.
The European traders used any means possible to obtain encourage the "production" of slaves as a commodity ( including direct kidnapping ), then shipped them as cargo ( literally ) to the Americas as the "middle passage" of a triangular trade ( European manufactures to Africa, then slaves to the Americas, then raw materials to Europe ).
In round numbers ( figures are disputed ), ~10 Million slaves reached their final destination ( ~1 Milion to the US ), ~8 Million died after enslavement but before reaching their destination. The activities in Africa to obtain the ~18 Million enslaved ( raids, wars, kidnapping etc ) are estimated to have resulted in ~8 Million more deaths.
The Aztec behaviour was, as you rightly point out, atrocious. While the Aztec get the name-check as the big Civilization of pre-columbian Central America, they were actually a relatively small group that dominated many of the surrounding states through conquest. But even then, possibly half of the Central American states were independent. From what I have read, their religion had a prophesy of impending calamity at the time the Spanish arrived, which they believed could be averted through mass human sacrifice.
You might disagree, but I would not classify these victims as slaves. They were "acquired" from the Aztec's vassal populations, and through attacking neighbouring states, purely for the purpose of sacrifice. As a result, many of the Central American states were ( at least initially ) happy to see the Spanish arrive.
Top prize for appaling historical behaviour, however, probably goes to Chinggis and his generals of the Mongol Hordes. In addition to the complete destruction of cities and slaughter of all inhabitants ( Merv, Nishapur etc ), they are thought to have caused the death of about 5% of the World's population, and destroyed 50-90% of population of certain resisting states. Not infrequently, states that surrendered were then forced to provide their troops as part of the Horde, participating in, and making them complicit in the next massacre.
Who needs evil races, when you have Humanity?