I think it is a deep mistake for players and designers to expect a 1:1 correspondence between gameplay mechanics and real life. In all our time gaming (tabletop / board game or electronic) how many innocent lives have we players all taken collectively? How much have we stolen? How many have we wronged?
If we are talking about fictional entities the answer can only be zero. These things do not exist. They are not real. Therefore no living thing can be harmed, and what kind of creatures are we if we are obsessed with the morality of deeds against non-living, non-existent things? At best we can be accused only of committing crimes of the imagination.
I agree with the point others have made that some will see your work as taking a position, and I feel we currently live in a time where being seen to be moral is perhaps more important than actually BEING moral. If position is a concern, I'd simply make it clear in the manual that this was historical fact and its inclusion is neither meant to condone nor glorify and leave it at that. Some people will nonetheless be deeply offended. It is their right to be so, and it is your right to ignore them. Let them create works that promote their own values. Maybe your work can serve as a springboard for a game about abolition or a deeper simulator whose mechanics plumb the depths of the psychology of objectification.
As an aside let me also note that a rising interest in injecting modern morality into gaming isn't in and of itself a bad thing. There are wide open frontiers to explore in that direction. But a game is not somehow immoral because it incorporates distasteful and even downright evil subject matter. With gaming we must resist the mechanism in our brain that correlates thinking about something as doing it. Thinking is thinking. Doing is doing.