This might surprise you, but I grew up knowing this. I always heard it was brother against brother and father against son. There were many Southerners who didn't resign from the US Army at the outbreak. One of the most notable is the Rock of Chickamauga, George H. Thomas. No ground breaking revelation here.
What most fail to take into account when looking at slavery as the cause of the WTBS (and it was A cause), is the depth of the potential economic upheaval. Although the leading Southerners' (politicians/plantation owners) arrogance and rush to rebellion was the main catalyst for firing on Ft. Sumter, their mindset must be considered. What was the plan for emancipation of slaves? What would become of their investment. Slaves were property and there was much invested in their upkeep and maintenace. Who would share the monetary loss if their property (slaves) was liquidated by federal mandate (federal subsidies?)? The nonslave holder had equal concerns that had to be addressed...a potentially flooded job market, by skilled and unskilled laborers, most likely willing to work for much less. This is where this issue isn't just about slavery, but economics. Answers to these questions would have been interesting, but both the CSA and the USA took the road to war before alternative solutions were found.
The social issues resulting from a caste-like society were complex. The country had been segregated (for the most part) since its inception. The forced and rapid desegregation of the South after the war was the main reason Reconstruction was so bitter. Probably the reason that it stayed segregated into the 1960s (still that way in many places throughout the country...North and South). Its a shame that these issues weren't worked out peacefully, 600,000 dead is a high price for poor communication and arrogance (on both sides).
The WBTS was a watershed event in OUR history. Trying to paint it as black and white (no pun intended) is an oversimplification of the most complex event/era in OUR history. It also keeps US from learning the causeS of the event, and robs us of the insight to better understand ourselves as a county. Blacks served on both sides, I'm sure. Blacks were kept at arms length by both sides, too. What was true then is still true today...you can make people live together, but you can't make them accept each other or get along, unless there is a co-willingness to try. Hopefully, we'll get there some day.