As I've read about the history of slavery, I've found it to be sort of a theme for slavery to be accepted as part of a society through many dead civilizations; If I'm not mistaken, it dates back at least to Mesopotamian cultures, where people would go into slavery to pay off debt.
I have heard that slaves were treated at their absolute worst in the decades leading up to the American Civil War, and given the vivid details we have on that, I wouldn't be surprised. Is this true?
In the American south slaves generally had a reasonable life expectancy and had children. Killing your slaves or working them literally to death was a bad use of resources compared to getting many years of work and the service of their descendants. Also importing new slaves became illegal, so slaves were not that easily replacable as a whole.
In other settings -Caribbean sugar cane plantations for example - the nature of the work done by slaves led to a generally short life expectancy due to disease and occupational hazards. in such settings, the most efficient use of the investment in a slave was literally to work them to death over the course of a few years.
It depends on what you're asking. If you mean the broad dehumanization of a people, there are parallels we're aware of, but they generally lacked the concept of race as we think of it. If you mean working conditions, well, mines in the Roman empire were as close to hell on earth as can be imagined.
Still, the labor camps and death camps during the Holocaust probably qualify as the bloodiest and most brutal slavery the world has ever seen.