The "vitriolic" demonization I see as pushback against creeping revisionism.
A lot of the soldiers who fought for the confederacy were poor and had no choice, and also didn't own any slaves. It seems appropriate to build a memorial for them, BUT, that's always been (and still is) the case for soldiers everywhere, and nameless grunts already have many monuments.
But the war was fought over who would control the economy in the West. One of those systems enslaved human beings into forced hard labor for life. You can't escape that fact, that only 150 years ago, you could murder a black child if she talked back to you, unless someone else literally owned her, in which case he'd sue, since it's property damage.
It is not some blood-libel on southerners, and during the Great Migration, plenty of slave descendants found different but equal racism in the north. No one's innocent here. But don't pretend that this system wasn't central to the war.
Those who fly the "confederate flag" (none of the ones you see were actually flown at the time), deliberately or not dilute the horrors of that system. You can find plenty of attempts and some successes at getting a rosy picture of slavery into school text books.
Why is it important to fight that? Because implanting the idea that blacks were "better off" under slavery winds up promoting racist policy today. You also can't deny that some icons have become symbols used by modern racists. What other meaning can you put to the confederate flag anyway? The heritage, like it or not, includes the enslavement of a people, and the willingness to go to war to preserve it. Would you really fly it in front of your black friends and not reflect a bit on how you can not literally shoot them dead, or hang them, if you felt like it, anymore? And order the survivor to bury the body?
I'd like to think that we who oppose public monuments can separate contemporary southerners and dead racist plantation owners, and I think we do most of the time. But that gets tough watching Trump rallies. But, we're trying to counter the image of a pastoral south innocently rising to self-defense that so many want to promote--not to attack southerners, but to remind people of the horrifying realities. I mean, southern slavery was some of the worst in human history. Shit, Roman slaves were part of the household, worked as accountants and businessmen, had certain rights. Blacks in the south before the war were sub-human. Pack animals, same as a mule.
Personally though, I'm glad to live in a country where you can fly an enemy flag legally. And one where people will argue and vote on taking it down (rather than beat your ass).