The quote above from Justice Taney demonstrates such a common human trait -- rationalization. To abuse another group of persons, one must dehumanize and/or vilify them. Most decent folk raised in a culture that extolls the value of individual liberty won't say, "Oh yes, I'm oppressing and brutalizing perfectly decent folk who are just like me, depriving them of their freedom because lining my pockets is more important than the principles we cherish."
As to the treason point, though, today we take it for granted that an independent state that voluntarily joins a union of other independent states is irrevocably bound to that union for all time, no matter what. At the time of the formation of the Federal Government it was not clear, up front, that once an independent sovereign state joined the union it forever relinquished its freedom to withdraw from that union. I believe this was a question that reasonable people could differ on.
Think about whether withdrawal from a political union is, in itself, morally wrong. For example, if the South had been able to dominate the Federal Government and (through legislation or Constitutional Amendment) mandate slavery throughout the entire Union, and if as a result some Northern States elected to withdraw from the Union, I question whether such an act would constitute "treason." Or you can imagine other circumstances where some states in a union are dominating and subjugating others, and the disadvantaged states therefore exercise their right to withdraw from what they voluntarily joined.
The real moral problem for the South wasn't "treason," in my mind, but rather the reason the Southern States chose to withdraw. Because, when push came to shove, it really was all about slavery.
Still, as I understand it, many (most?) Southern whites did not own slaves. Some no doubt really did believe they were defending themselves from the armies of the invading Northern States, and felt they were protecting their homeland. Again, such a concept in and of itself is not morally repugnant. What is repugnant are the underlying reasons the Southern States withdrew from the Union (the shifting balance of economic and political power, ultimately all surrounding the issue of slavery).
Anyway, I can't speak for the views of some Southern whites today. I grew up with the view that we (the North, the Union) were the good guys and Abraham Lincoln was a hero, the President who freed the slaves. I imagine in the South the schools taught a different narrative decades ago. Focusing on Northern domination, the rights of their sovereign states, and self-defense, while downplaying and whitewashing the issue of slavery.
