There are arguments to be made that the south saw slavery becoming less fashionable in Great Britain and were into rebellion for that reason. They knew staying in Britain meant slavery going away eventually, and might have thought they could delay that better as an independent nation. Remember that the original system after independence was that state governors largely had their own little fiefdoms, and the federal government had very little power.

And they would have been correct; Britain did ban slavery before the US. First for Britain itself in 1807, then broadly in it's remaining holdout colonies in 1833.

And they did so without a civil war.