And, you can attempt to mitigate their actions in certain ways that are not wholly invalid, but the fact remains that they were traitors, actual treasonous traitors, to the United States of America.
The whole lost cause thing is pretty disgusting. It was, in essence, a set of oligarchs with a business model (slavery in cotton farming) that they attempted to make policy of the United States, at least for them, if no one else. The vast majority of southerners were a lot like magats: very little to gain for supporting their oligarchy leaders, but support they did, often with their lives.
And the oligarchs were sore losers who spread their sore loserness among more common or garden southerners, creating a false nobility to the whole fucking mess. And most of them were virulent racists that encouraged virulent racism among the masses, although the masses had more in common with slaves than the oligarchy.
And the lost cause mutated into Jim Crow, meaning the peculiar institution would continue to haunt this country for probably the rest of its existence, certainly to the present, at any rate.