Subverted in Rome, where the protoganist Lucius Vorenus is shown trading slaves though it can also be because most people in the show are Jerkass.
Granted, that same protaganist also murders his own wife for no good reason, causing his children to hate and fear him for the entire rest of their entire lives.
Or BOTH of the above are neither Villainby Default nor Grey-and-Grey Morality (especially since Niobe commits suicide, she isn't murdered) but instead are examples of accurately-portrayed Values Dissonance. Slavery was legal and normal in Rome (and unlike in the pre-Civil War US, not based on race or considered a permanent status), and by Roman standards Niobe had committed a capital offense by committing adultery with and having a child by another man while Vorenus was still alive, whether she knew he was or not.
This doesn't seem to have anything to do with the trope. It's just Values Dissonance.
Removed:
This doesn't seem to have anything to do with the trope. It's just Values Dissonance.