It's a distinction with no difference. Yes, they wanted to form their own country. Why?
Slavery. If every answer you give leads back to "slavery", then the correct answer is "slavery." It's obfuscation to avoid that answer.
And they had dreams of expanding. They didn't just want to be left alone:
And remember when you parrot Lost Cause mythology:
You're too optimistic, I think. For one, even while it was officially illegal, the South continued to use slave labor through the 1940s in the form of contract leasing, with the vast majority of the black population who weren't in that situation so constrained by a web of laws and extralegal threats (like lynching) that though nominally free, they weren't really free. And the South had already begun diversifying its slavery practices by the 1860s. They were already using slaves to mine for ore by the end of the war; the foundry, arsenal, mines, and furnaces in Alabama, increasingly run by slave labor as the war came to its end, became integral to the Confederacy's ability to make arms.
This was the basis for a later shift to industrial slavery, which was more brutal than the slavery that preceded it. In the first two years that Alabama started leasing its prisoners, 20 percent died; 35 percent in the second; 45 percent in the fourth. This practice was wildly profitable, bringing tens of millions of dollars into state government coffers and creating the single largest revenue for Alabama. Even today, though far less brutal than it was prison slave labor remains widespread.
And if the South felt like they too large a slave population to control, I think I'd sooner expect genocide than abolition. Though I imagine it'd simply lead to much more brutal practices in which slaves weren't viewed as holding as much value individually and were worked to death.
Plus you have to consider the possibility that the South would try to further expand south, as I mentioned earlier in this post. I think that it's just too optimistic to assume that slavery would've simply faded away.
