“So what? Slavery was legal in this country under the American flag.”
Yes, and then a Civil War (you probably call it the War of Northern Aggression - I prefer War of Northern Victory) was fought, under that flag, in part to end slavery, and which, in fact, ultimately resulted in the end of slavery under the flag.
By contrast, the traitors who fought (and lost) under the Confederate flag were fighting for a cause that sought to protect slavery. Slavery under that flag ended only with the defeat of the flag and the cause it stood for.
Arguing that the two flags are similar with respect to slavery is pure ignorance.
“By contrast, the traitors who fought (and lost) under the Confederate flag were fighting for a cause that sought to protect slavery.”
So, if a State in the future wanted to secede from the Union for some other reason, would that be okay with you or would you be motivated to put yourself at risk to stop them?