There are always more than two options, especially when making assumptions. You don't have to assume guilt or innocence.

Slaves were seen as smarter versions of working animals or, in some cases, pets. We know from the historical record that some were horribly abused and some were treated as well as hired workers. Ultimately, it doesn't matter how well or badly an individual master behaved--even the most benevolent form of slavery is a denial of our basic human rights.