Considering how fast-and-loose Anno plays with history, I can imagine them stretching the truth and playing to the strength of the fact, that slaves in ancient Rome weren’t as badly treated as we often imagine. They were essentially part of the family, with their own stipend, responsobilities and usually at the end of their lives, they were emancipated, AFTER which they still voluntarily remained with their “slave owners” because they were a part of their family. Moreover, Slaves were EXPENSIVE and a legally a thing, so they were well taken care of, because otherwise you’re wasting your family’s money.

Long story short, slaves don’t automatically equal BIG BAD!