In a honest and realistic game about triangular trade, the player has to be able to trade slaves, and trading slaves has to be a major part of any good strategy (because there aren't enough other commodities that can be exported from Africa). This gives the players a choice between being evil in the game's fiction and losing the game.
If you don't want a grim/educational game in which slavery is a moral issue or a callous game in which slavery is appreciated as a smart business plan, the main way out of the dilemma is not making the game about ship trading after all, to avoid the use of slave trading as a move within the game. For instance:
- The popular Puerto Rico boardgame is about developing an American colony with city facilities and plantations. The masses of slaves working at whatever the players build are completely implicit: "workers" in the game arrive by ship in small numbers, and represent (with some ambiguity) European emigrants and big shots.
- A game about pirates robbing random ships would make the players not responsible for loading a particular ship with slaves, and rules about humanely and usefully recycling captured slaves as pirates, colonists etc. instead of selling them might be reasonable