@Fari
If you want to look at it positively, the MC taking care and treating certain slaves well could be considered as idealism, rather than them actually supporting slavery, because they know those people would still be slaves until death or freedom (which anyone else very rarely grants, let's face it). Especially if they're of the type of slave that's either cursed or wounded, and would pretty soon either be disposed of or experimented on.
Then there's also the different kinds of slaves.
Debt Slaves, Crime Slaves, War Slaves , Regular (Captured from somewhere) Slaves.
2 first are entirely logical, right ? 3rd one is understandable, right?
So if anything, if the MC buys slaves from the first 3 groups, they could be considered as 'supporting' society & law in a way.
It's only the last group that tends to really show the bad sides of humanity, because that's either because of jealousy, greed, envy, religion, politics etc. anyone who's different from the people enslaving them.