"It's impossible to completely uproot all evil!"
Well.........Perhaps consider this: if you guys have a society in which slavery is legal in some aspects, maybe it creates an economy in which "Cheap / free human-resources is a requirement",
Which can easily lead to a dystopian state of affairs. It may be a naive thought, but as long as you minimize every single external factor that can and will encourage people to act like animals, you're creating a situation in which you do not have to waltz around and uproot "bad fruits". Which is another reason why, for an instance, "the war on drugs" was so fucking stupid.