Slavery is wrong, period. However, saying the Confederates were "Devoted" to maintaining slavery is a statement that directly leads to false connotations, mainly that the North was some kind of moral deliverer who wanted to free the slaves (Nevermind that the North didn't abolish slavery either.)
Plus, while I absolutely deplore slavery, I think most of the other CSA positions, mainly reduced Federal governmen involvement, were correct. Supreme Court Justice John Marshall and his court basically manipulated the constitution to increase Federal power, which increased the wealth of the North. The South, rightfully, opposed this idea.
But the main reason I use the CSA flag on occasion is to simply spread the message that the right to secede is not, in fact, banned, and that the CSA were no more traitors than the Patriots.
Plus, horrible things were done in the name of the US flag as well, but we still patriotically defend it.
And finally, while I absolutely agree slavery is disgusting, it wasn't always like it is portrayed, many slaveowners had familial bonds with their slaves. It was big plantation owners that really abued them [assumption being only big plantation owners abused them, added by Hobbsyoyo]. And for the ones who were treated nicely, due to racist attitudes at the time, short of going to Canada, there was no better option for them.
In short, while slavery is completely evil, there's a bit more to the whole thing than you know. Not that I really expect someone from Britain who doesn't have a serious academic interest in American History to know the details. A lot of Americans falsely believe the same things you do about the CSA.