The Civil War was fought because of secession. The underlying reason, for the Confederacy wanting to secede from the Union, was they could not accept the abolition of slavery. One of the underlying reason for that does get into the financial reasons that the southern plantation owners did not think they could produce a sufficient profit if they didn't use slavery as their "engine" of commerce. But the declaration they put in their Cornerstone wasn't about economics; it was about their fundamental belief that the Negro was inferior and deserved to be a slave.
The Cornerstone of the Confederacy is about as far away as you can get from "All men are created equal and they are endowed by their creator with certain inalienable rights, and that among them are life, liberty, and the pursuit of happiness."
Treason is a strong word, but when you forcibly rebel against the prime principle upon which this country was founded, I guess it fits.
History is rarely simple and the causes of the Civil War are complex. Anyone who says slavery did not play a major role is in denial or uninformed. However, the issue was not abolishing slavery in 1860. Not one of the 4 major candidates for President in 1860 ( Lincoln, Douglass, Breckinridge and Bell) campaigned to abolish slavery, the issue was expansion into the western territories. The balance in the Senate was the desire of the South as they could never achieve balance in the House. If slavery was not allowed to expand into the west then the Senate would be lost as well. Should this occur, then eventually the abolitionists would have been strong enough to move against slavery. Tariffs were also a big issue. The large tariffs the national government passed to protect Northern industry harmed the Southern economy, which depended on exports, particularly cotton.
There is no denying that most Southerners looked down on persons of color. This was true in the North as well. These attitudes carried on into the 20th century. While the South had Jim Crow, don't forget some of the worst violence about busing and integration occurred in Boston. Other Northern cities had similar problems.