The civil war was not primarily about slavery...it was about the south feeling like it had no power and its way of life was being threatened. The South was losing representation in the House and many southerns felt like the North would use its power to weaken the South. Of course slavery was part of it, since the South was threatened by northerners passing legislation ending slavery and crippling the economy of the south. The northerner's aim in the civil war was not primarily to end slavery, since abolitionists were a small minority and most had no opinion at all on the matter. It was mostly about preserving the Union, because the Unionists believed that the Constitution stated the the Union derived its power from the people, so only the people as a whole could decide to break it up.
I believe the South had no legal right to secede, since I agree with the Unionist interpretation of the Constitution. The confederate flag should not be banned, but people should realize that it doesn't only represent Southern pride, but also racism.