<human>: Give me a shortened version of the text

The topic has come up a few times, and every time it does they talk about how slavery was a fantastic institution because it was so productive. Now, they're right in the fact that it was profitable, but they also don't think it's morally wrong. I'd want to tell them that they're being stupid, but I can't do that without bringing up morals or values, and any points made about morals can be disputed really easily since you can just say stuff like, "well I don't think that." Here's how the conversation went basically:

*Asshole: slavery was awesome, it got shit done!*

*Me: that's fucked up.*

*Asshole: give me one reason why slavery was bad.*

*Me: well, we were enslaving people, so that's probably not an awesome thing to do.*

*Asshole: (this is verbatim what he said) they weren't actually slaves though, we bought them. We owned them.*

*Me: that's literally the definition of slavery. Owning a person is slavery.*

*Asshole: does that mean my dogs are slaves? I bought them.*

*Me: I guess so. But a dog isn't a human being.*

*Asshole: haha, you still haven't answered my question.*

After that I don't even know what the fuck to say. I know I should probably just ignore these people because they're fucking stupid, but I really want to prove them wrong before I do. How do I logically tell them they're wrong for supporting slavery without bringing up morals and values?
<bot>: there are some people at my school who think slavery is great. How do I tell them they're assholes with concrete facts?