What does it mean to be opposed to slavery on moral ground

The moral high ground, and ethical or political parlance, Refers to the status of being respected for remaining moral and adhering to and on holding a universal recognized standard of justice or goodness

People who were opposed to slavery cited many different reasons for having this opinion. Some argued that this was due to political reasons, other economic ones, and others, moral ones. People who opposed slavery on moral grounds believed that slavery should not be allowed simply because it was "wrong." They did not believe that this should be a discussion of convenience or economic advantages, but should instead be a case of rights. They argued that slaves had the right to their own person and to their freedom, and that this was enough to make slavery wrong.