Just how do you know that? Probably you learned this from some African Studies Communist textbook author.
Some slaves were treated very humanely, some savagely, from the beginning of time. Making generalities about individual experiences is not being very open minded.
The poster that you are replying to never said such a thing. Nobody here is denying that some slaves were treated well.
There are biblical parameters around how slavery is to be conducted. Ultimately, it's about being a bondservant and, in the end, all bonds are nil after 7 years of serving one's master, having fulfilled one's bond. Now that slave or bondservant is welcome to leave or continue to work for his master.
I think it's pretty clear that these parameters were not followed in the west, whether slaves were treated well or not. Obviously there's more to this subject than I've said here, and we can go into the biblical case for slavery if you like, but again, I'm not convinced that these parameters were followed by anyone, perhaps with the possible exception of Israelites before the Babylonian captivity, but I'm not certain on that one. Obviously they had their own problems with disobeying God too, so I'm sure they broke these laws too.
But I digress