But still it's easier to understand how the South could rebel when 25% of its people derived direct benefit from slavery and countless more derived indirect benefit.
I've always recognized that slavery was a major cause -- I'd say the root cause -- for the sectional differences that led to secession. I don't infer from this, though, that Confederate soldiers where fighting for slavery, especially not the majority who came from households without slaves, and not even slaveholders just as Lee. When it came to the war itself, I think defending what they considered their homeland was a more important motivation.
I'm quite willing to denounce slavery (as Jefferson himself had), but am not willing to treat with contempt persons whose actions were understandable in the historical context of their societies. The Mason-Dixon line didn't divide good from evil in the United States, and to interpret that war in simplistic terms is neither accurate nor fair to the persons involved.