The Development of Moral Reasoning

The cathegorical imperative: Act in such a way that the maxim of your action be instituted as a universal law of nature. (I.K.)
Another form of it is "Man is never merely a means to an end, but always an end onto himself."

On the assumption that we could not even have morality unless we had authonomy (the freedom to choose one course of action over another) - in order to preserve myself as a moral being, I must be sure that others will not use me as a tool or an instrument or else my actions are not the free actions of a moral being, rather they are the mechanical actions of a kind of device.
So anyone that would use another as a tool or an instrument is (in principle) denying that very authonomy on which morality itself depends.

The wrongfullness of slavery on this account is grounded into the fact that to enslave another is to eliminate the very possibility of a moral life. So whatever you might say about an institution like slavery, you couldn't say that it's morally right. The very maxim that would warrant slavery or would justify slavery virtually eliminates the cathegories of right and wrong.