The option to buy slaves early on makes lots of sense for the plot. He's new in the country, he's following customs, he's outright given one slave and an ex-slave is his secretary. Then he learns more about how slavery is done, starts to dislike it all, meets fucking Trello, etc etc.
I agree that there should be the option to switch from mean slaver route to nice slaver route, though. THAT would complete the bit above. Progress in Lin's storyline far enough, or something like that and you can be like "oh shit, my bad, I bought into this whole corrupt system. Now that I know more about you and your culture, I shouldn't have been doing this shit."
Actually, wait, looking at the state of the world today, that's super unrealistic, aint it?