If Nazi Germany would have won the Second World War, and if they would have preceeded to brainwash the entire world to believe their ideology, and if they had killed everyone who was resistant, so that in the end, everyone who was left were a convinced Nazi - Would the Holocaust still have been an evil deed?

Likewise, if the Confederacy would have defeated the Union in the American Civil War, and if they then would have set their eyes on the world, and if they came to dominate it, and killed everyone who considered people of color to be fully human, and/or opposed enslaving them - Would the transatlantic slave trade and following chattel slavery still have been wrong, something that ought not to have be done?

What do you think is the worst thing that humans have ever done to other humans? Maybe one of the above? Maybe the genocides after the Ottoman empire fell? Or the Nanjing Massacre? Perhaps what Leopold II did to the people of Congo? Or recent events in Ukraine, the Middle East or Africa? Maybe something more personal that someone did to you, or to someone you hold dear? Take the worst you can come up with, and consider, would that still be wrongful, even if all remaining humans thought it wasn't?

If you come up with the answer "no" for any of these questions, then does that bother you?

If the answer is "yes" to either of them, then what does that mean? Where does morality come from? Where does right and wrong come from, if it doesn't require a human to be around to think of it as such? Where is it sourced, if not in human thought?