My dude, most of the states that seceded specifically listed slavery as their main reason in their articles of secession. We don't just have it in the words of the victors, we have it in the words of the losers, too. It's an indisputable fact that the south seceded primarily to protect slavery. Likewise with the Nazis and their genocidal twattery. They kept meticulous records of their own atrocities, so once again, we have it in the words of both the victors and the losers.
These are hard facts. They're historically undeniable. The Confederates were slaver scum. The Nazis were genocidal monsters. They were both ultimately proven to be fucking losers. If any of this wasn't true, you'd bring some proof, but you don't have any because it doesn't exist.