So you are trying to argue that slavery is a RIGHT? This looks like and argument of guilt by association. Authoritarian is seen as bad, by giving the abolishment of slavery the label of "authoritarian" gives of the idea that you want to associate it with being bad.
If having a law that restricts one’s ability to do something is “authoritarian” then any law is authoritarian, because laws, by definition, determine what behaviour is and isn’t allowed within a society. On that note, morality determines legality, not the other way around.
Slavery means that, if you're rich enough, you should be allowed to revoke the rights of others. This is refutable at so many levels. If someone were to "willingly" agree to give up their rights, then just you're just taking advantage of someone who was born in an unfavourable position and have no other choice other than to accept (and maybe not starve) or starve.
No... Capitalism is when a handful of individuals own the means of production and have full authority on how they are used.
Communism is more akin to when the workers decide what to do with the means of production they operate.
You got this literally backwards.