It is a symbol of oppression and slavery.
It's a symbol of people standing up and fighting for what they think is right.
An ideology not too dissimilar from that which founded the USA itself.
The Union wasn't fighting for the explicit abolition of slavery but to maintain a cohesive national identity and enforce federal powers.
The conflict was indeed largely inflamed over the abolitionist movement, but was truthfully about whether or not it was reasonable for populous northern states to undermine the industry of Southern states for the sake of anti-slavery ideology which was far from popular at the time with the common folk.
There is maybe some reasonable argument to the idea that this wasn't fair political representation.
The idea of the South as a racist boogeyman, is historically fatuous.
If we're going to hold it accountable to modern standards of slavery and racial equality, why then isn't the entirety of pre-abolition human history equivalently accountable in this.
How long was the American flag itself complicit with slavery?
They weren't fighting to keep a people down, but to defend their own way of life.
Simply because some people might use it as a symbol of racial hatred, or that others may interpret it as such, doesn't mean it should be banned or suppressed for obvious reasons (freedom of speech).
Even from the political perspective of treachery or sedition, the very nature of a democratic and representative government mandates that all political perspectives be allowed free expression.
It isn't the idea or motivations of rebellion that should be punished, but the act, especially considering the rebellious origins of the state itself.
What right did the Union respectively have to deny the agency of people who felt unrepresented by their government?
It is realistically a grayer ideological conflict than more people are comfortable admitting, and still reflected in modern political discussion Big Government(Federal Power)/Small Government(State Power).
The historicity of the flag, and its political symbolism dramatically outweigh anybody's personal feelings in the matter.
That said, you know, I wouldn't fly it over any currently officially active government bodies, considering the matter was politically resolved even if it ideologically wasn't.
We can't enforce any form of law if people aren't willing to participate in a unified society.
You can and always will be allowed to fly it anywhere you want though.