Lifelong Southerner, born and raised within 20 miles of the SC State House. I grew up hearing all about the 'lost cause' and 'state's rights' stuff being what kicked off the Recent Unpleasantness. I bought into it- until I learned to think for myself. That included reading the Articles of Secession for not only SC but other Southern states. Whitewash it all you want, slavery was THE proximate cause of secession and all the terrible things which happened afterwards.
The Southern states formed their own central government, then designed their own flags including the Southern Cross. (That's its real name, not the Stars and Bars. That name refers to the flag of the Confederacy, analogous to the US flag.) We all know how that went; the South lost, Lincoln emancipated the Southern slaves, then the long hard work of rebuilding a nation began. Like any other conquered nation, the various Confederate flags came down and were replaced with US flags.
So later on, during the desegregation fights of the 1960's and 70's, several Southern states incorporated the Southern Cross in their flags or flew it from their capitols as a big middle finger to the US government. Now stop and consider that for a moment; the sons and daughters of slaves who were held under that banner see it come back again as a symbol of 'separate but equal' being the law of the land, i.e. the white people think you aren't as good as them and they won't let you forget that. I bet if it were you, you'd want to see something different happen too. But no; there it is, flying from the state capitol dome, a symbol of tacit state government approval for all that the banner stood for back in those days before emancipation.
Yeah, it needs to be removed from the State House grounds. Someone wants to put a flag sticker on their bumper, fly one from their private flagpole, display one as a memorial to one of their ancestors? Go right ahead. Just don't expect the government to display it.
Oh, and about the US flag flying over slavery, here's a few facts: there was a LOT of arguing about the future of slavery in the newly minted US back when the Constitution was written. In true political fashion, that can was kicked down the road; read up on the arguments over the various new states being free or slave. Hint: the Mason-Dixon Line is not just a fanciful term for the dividing line between North and South. Also see the Missouri Compromise line.
The Northern states had all agreed by 1804 to end slavery in a 'phased out' fashion; the Northern states not returning escaped slaves is prominently mentioned in the SC articles as a proximate reason for secession. So the US flag flew over a half slave/half free country. Emancipation of the slaves and rebuilding of the country happened under that flag as well. IMHO that gives it a free pass.
By the way, my dad was into genealogy in the last several years of his life and self published a book about our family. I had an ancestor who was somewhat famous around these parts for winning a battle in the area of Horrell Hill, SC, there are reprints of old newspaper stories telling of his daring and courage. I can honor his courage while still acknowledging that the cause he fought for was wrong. My dad and I used to argue about the flag fairly constantly; I still see and remember his good points while acknowledging that no he wasn't perfect.
