I think you can and should use it. You’ve explained how slavery was an important aspect of ancient Roman life and warfare, and if you want to keep your game historically accuate you should include it. You may give the MC an option to express their opinion on capturing and selling the POWs and possibly a choice on how they threat them.
In another WIP set in Ancient Rome, When in Rome, the MC owns a slave by default and can become a slave trader’s assistant. They also get choices to express their attitude towards slavery and treat the slaves better or worse. And in many military-themed games and WIPs the MC can engage in things like looting, pointless killing or extorting things form the defeated.
@TechDragon610 have you played Choice of the Vampire? It is set in the Old South, the backgrounds for the MC include being a former slave and being a slave-owner, they can become a slave-owner mid-game and if he’s white and male the MC can even join the KKK. Several NPCs are either slaves or slave owners.