Note: I'm not from the south, I don't live in the south, I don't approve of slavery, etc... etc...
Absolutely. The Confederacy was founded upon the southern states' "right" to continue the use of slavery.
Somewhat, but that's a bit of an oversimplification. While slavery was a major part of it, State vs Federal control was a major part and outlawing of slavery was more of a final straw for the South.
Many people who fought for the North held slaves, and many people who were anti-slavery went and fought for the South. On its own, slavery was a big issue (partly because a major part of the South's economy depended on slavery), but only led to war because of the southern states were already feeling marginalized by the Northern states' control of the federal government. It became a war of holding together the union of the states as one nation (modern USA) vs having a bunch of more independant states tied together economically (kinda like what I imagine the European Union might be like - but I don't know how the EU operates).
Slavery became the major issue of the civil war - but it wasn't the sole cause, and on its own very likely wouldn't have led to war.
If people rally around the confederate flag as a cultural symbol, I've no problem with that.
But when that culture includes racism, and people rally around the flag as a symbol of that racism, that's a problem - but still within their freedom of speech (removing the symbol won't remove the racism). I'm pro-Israel, but Americans still have the right of getting a swastika tattooed on them. If a swastika was added to a government building, however, I'd have a huge problem with that.
And that's my biggest issue with the confederate flag: when the state governments fly them on government ground. While they may or may not be making a cultural statement, it can easily be perceived as, and probably often is, a racial statement as well. It practically gives government approval of racism - a smile and a wink at racism.