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.