Another thing I noticed is getting more common among RPG Horror Stories. When once it was common to see entitled players complaining the GM is not running the game like Matt Mercer runs on Critical Role, I have lately seen quite few stories where problem GM tries to use that to deflect criticism. It's usually the type to be acting creepily towards women, both in and out of game, enjoying juvenile, overtly edgy humor and/or insisting of all kinds of bigotry for "historical accurracy". And when the players confront him (as it's almost always a guy) about it, he's going to say something like "Stop sucking Mercer off, this is real D&D!" or "Go play at Matt Mercer's table, if you don't like it!".
While, as usual, there is possibility these stories are fake, I can see these being true - the kind to engage in those specific behaviors is also the kind to grab on buzzwords or try to twist real problems to deflect criticism.
Idk, I think that might be a bit of an overreaction and a missed opportunity. He has a good point about being from that town and slavery being a normal part of his life growing up. That could've turned into an interesting in-character exploration of cultural moral standards: genuine confusion about what's wrong with being a "good" slave-owner, maybe a conversation about how easily freed slaves are re-captured, it could turn into a whole revelation for the barb that culminates in a quest to dismantle the entire slave trade.
Obviously we're missing some context, and it's possible that the player exhibited problematic behavior, but personally I don't think the scenario is itself that bad. Just sounds like a barbarian from a slave-trading society role playing their character. Some would argue that eating meat would be likewise incompatible with a good-alignment, but our culture sees no fundamental moral objection to slaughtering animals.
That can only be possible when the player knows that slavery is evil, but is role-playing a character who doesn't know it/has never really thought about it.
But the bit about wanting to be a good slave owner like a pre-civil war slaver, and that someone can only be good or bad relative to their culture, implies that it was said out of character. The fact that a person really believes that there is a difference between good and bad slave owners (and specifically mentioned the pre-civil war era, lol) is a massive red flag.
First of all, it's stupid: just because slavery exists in your society, you don't need to be a slaver. Good people can exist in a corrupt society as well. If they didn't, we'd still have slavery today. Heck, one of the most famous DnD characters is a dark elf who cut ties with his people to fight for the Good (Drizz't). If slavers are brought up in a good campaign, the obvious conclusion would be to stop them, not to take part in the evil system.
There's also the fact that, if the campaign is specifically asking for good-aligned characters, nobody would expect someone to "well, akshually slavery can be good" them. Like, maybe it is (it's not), but you're explicitly not playing a good character, so why are you doing that? Join any other group out there. This group probably doesn't want you to shift on them the burden of discussing why drowning puppies in the well is a bad behaviour, while you're drowning those puppies.
I could also point out that (1) the fact that he doubled and tripled down on his intention of owning slaves, and quit the table because of it, is kind of moronic, and (2) depicting the girl of the party specifically as a "screaming queen" rings of misogyny as well.
Also, I'm not really going to give the benefit of the doubt to someone whose idea of a good character is a cosplay of a pre-civil war south american slave owner.
It's an interesting idea to explore, but also something I think should probably be explicitly mentioned and discussed beforehand as a character flaw that you intend to be fixed.
Not something you drop on the party offhandedly and expect them to be chill with.