That Rean (male hero) dude... *sigh*
First of all; he's a "foreigner" in a foreign country with different customs and laws. Sure he's allowed to be disgusted by slavery but... It's not his house, he's a guest in someone else's. Therefore he has no right to judge others from that world just because he's an idealistic fool without any sense of propriety.
Second; he's spouting bullshit about the mc without any proof, that is literally slander, defamation and character assasination. Which is illegal on Earth as well. (I assume there are laws in that world against slander, defamation etc.)
Third; He doesn't really care about those women, he just wants to get in their pants. If he really cared about those women, he would ask their opinion first. And if they want to stay with their master, who is he to say otherwise? Please just kill him off, I lost a good portion of my braincells because of this dumbass.
Last but not least; the guy claims that there is no slavery on earth but that statement is incorrect. Sure slavery is officially abolished for about 200 years but it didn't disappear, just moved from the mainstream to underground. There is still human traficking today, so that guy is ignorant as fuck and is basically a keyboard social justice warrior that got lucky to be issekai'd. In other words; He is a slandering hypocrite, that is willingly ignorant about facts and makes wild claims before proving it.
Also to abolish slavery in that world, the people of that world must want it to abolish it. If they don't want to abolish it then nothing can be done about it. Unless he wants to create a dictatorship and force the people of that world to abolish it, but that would make him even worse than to just let it happen. And even on Earth, we have thousands of years of history with slavery, it's only been 200 years, so I consider that recent history in comparison with the rest of history. That guy being triggered cause of slavery is just his "political correctness" and his ideals that he wants to shove down everyone's throat.
Sorry for the wall of text but I kinda got triggered by this dumbass xD
edit: Vega's statement; "Mr hero has a strong sense of justice..." Yeah, no. He has no sense of justice, it's just his own ego that can't handle the fact that another man is more popular than him, he's just using the slavery thing as an excuse. Blergh