People justify conscription all the time, which is a particularly bad form of slavery. There is a difference between someone being evil and someone being wrong.
No, I don't think there is. Anyone about to do evil will cook up a batch of kool-aid to transform the act of evil into a "mere" act of being wrong, and the "mere" act of being wrong can certainly lead to being evil in short order. I do not think this distinction exists with anywhere near the level of clarity that would be required to redeem the concept of moral relativism.
Slavery still exists today. It's everywhere we just choose to ignore it for the small chance that one of us will become insanely wealthy. It's just greed. Everyone is capable of it in any time.