What does the rebel flag symbolize? Possible answers:
a) heritage
Okay. So what? Heritage isn't synonymous with "virtue," or "value," or being on the right side of history. If one's heritage is troubled--or simply in the wrong, or representative of a woeful moral grievance--then understanding one's heritage is one thing. Celebrating it is something else.
b) history
Indeed, and see above. An understanding of history doesn't mean we necessarily celebrate it.
c) freedom
For who? From what? If the rebel flag is used as a symbol of "freedom" we need to address these questions. If it's freedom from, say, Washington, then the flag symbolizes treason today as surely as it symbolized treason in 1861. But cries of "freedom" and "liberty" come loudest these days from people who claim to be "patriots" who "love their country." You can't have it both ways (unless, of course, the "country" you really love doesn't exist anymore).
So even at its most benign there are few compelling arguments for defending the rebel flag or what it supposedly stands for. What the rebel flag truly does represent--a failed rebellion led by white supremacists hell-bent on maintaining their superiority over an entire race they held in bondage--should persuade any thinking person that the rebel flag is nothing but a icon of hatred, racism, and division.
There is no good argument to be made for it.