I think the first thing someone should understand about that flag is that it wasn't about "slavery".
In their own words, they thought it was...
http://www.civilwar.org/education/history/primarysources/declarationofcauses.html?referrer=https://www.google.com/
I don't guess you can hold a major fraction (the actual majority in two states) of one's population as a possession and de-facto agricultural implement without that having a major effect on one's outlook.
The overwhelming fear was, in their own words, that an an actual majority of non-slavery states inevitably would mandate abolition. Abolition and the implications thereof were regarded as catastrophic.
I blame cotton more'n anything, cotton production exploded across the South in the Nineteenth Century and became the major cash crop, ALL the COnfederate leadership had made their fortunes in it, couldn't imagine a South without it. And cotton needed slavery.
I'd say JMHO, but it ain't.
Birdwatcher
