I think this quote from Andrew Ryan sums how this would not be a fun game if it existed.
A man chooses. A slave obeys.
By making your character a slave trader, you are condemning him to be a bastard. In a serial killer roguelike, you could just murder people who are corrupt or even other murderers. In the serial killer roguelike, you can choose to be a bastard, and straying from the open moral path seems like an actual choice, even if it isn't there in the first place. You just need an illusion of it. Serial killers have never been endorsed ever, so we feel that we are actually doing something, and we will be remembered for it. This is the basic point of sim games. You'll make the best (noun) ever and always be remembered!
However, in this non-existent slave game, in history terms, you'd just be a comma at best, the dot on an exclamation mark used by a person who hates you at worst. You won't even be remembered as the worst slave trader ever. They'll blame the society and the laws that created you, not you yourself. You'll be forgotten like the rest of those bastards who made human beings that could think and accomplish great things into property to abuse. You will always be brought to justice in the end, and made to pay for your crimes against humanity.
Whoa, got a little steamy. Sorry.