By what authority, when slavery was legal and even supported by the people in the South, did humans have the right to claim it was immoral.

What argument would you make that slavery violated a "human's" rights, if a human's rights are determined by the majority or government?

Who decides on those rights if not the majority or government? For Christians and others that believe in a higher authority than "the majority" or government and believe that higher authority grants every human... the human right of "liberty", slavery is wrong.

Doesn't matter what the majority think or government. So, what would your basis for a morality be, if not determined by God, government or the majority.

You have a right to an opinion, but such a poorly constructed response does not provide any evidence that you have one on the basis for morality. Keep in mind, other societies in the past had no issue with human sacrifice. Other majorities practice cannibalism. Governments like the Nazis and even China today justified genocide.

By whose authority are those acts wrong?