“Were slaves mistreated? Sure, some were. But they were the exception, not the rule.”
They were human beings who were owned and forced to do hard labor, and their children were taken away and sold.
You have an odd understanding of mistreatment.
You have seen too many episodes of "Roots."
As other posters have pointed out, slaves were expensive and vital to the success of the vast plantations of the South. It would make little sense to spend that kind of money on something so important, then take it home and break it.
Now, admittedly, the institution of slavery is, by its very nature, an offense against humanity. But the overblown image of slaveowners as sadistic brutalizers is largely fiction cooked up by abolitionists to outlaw the practice. Many slaves were better off before they were freed than afterward, thanks to Democrat policies that made them second-class citizens and left them to fend for themselves.