It might have been legal and even accepted as normal by many but slavery has always been morally wrong. How could it have ever been right? How could any man ever have had the right to own another man? Who gave him that right? The same goes for murder. How could any man ever have the right to steal another mans life that did not belong to him? Who gave him that right? Can you think of a circumstance in which it would be morally right for someone to take your life from you when you were not harming them?

A right could be defined as any action that is taken that does not cause harm to another sensient being. If the action you take does cause harm to another being (that is not threatening or causing harm to you) then you do not have the right to take the action. It's that simple.

Moral relativism allows people to excuse whatever behavior or action they see fit as long as they believe in thier heads it is ok or that the people around them believe its ok.