I think your denial of history stems from your 21st century sensibilities. The slaves and freedmen that fought came from countries where slavery flourished and they were sold into slavery by their own people. Even if they themselves didn’t wish to be slaves, the idea of slavery in itself, wasn’t as abhorrent to them as you would believe. It was a fact of life and not a moral issue.
Most of the slaves, even while in servitude and without any real freedom, were not abused and they lived better lives that they had when they were free men in their own country. This is no excuse for slavery. It only illustrates the mindset in which they would want to take up arms to defend the threat the US government made to their own security and livelyhood within the southern states.
I have no denial of history at all. The disgusting libertine principles of the Confederate democrats were to uphold slavery and to usurp the rule of law for all.