"Our new government['s] . . . foundations are laid, its cornerstone rests upon the great truth, that the negro is not equal to the white man; that slavery—subordination to the superior race—is his natural and normal condition. This, our new government, is the first, in the history of the world, based upon this great physical, philosophical, and moral truth."
This odious reasoning eventually led to the ideology of the Klan and post-war segregation, with enduring harm to America and the South.
"If centralism is ultimately to prevail; if our entire system of free Institutions as established by our common ancestors is to be subverted, and an Empire is to be established in their stead; if that is to be the last scene of the great tragic drama now being enacted: then, be assured, that we of the South will be acquitted, not only in our own consciences, but in the judgment of mankind, of all responsibility for so terrible a catastrophe, and from all guilt of so great a crime against humanity." -Alexander Stephens
What did Stephens say about why the North was fighting?
“Their philanthropy yields to their interests. Notwithstanding their professions of humanity, they are disinclined to give up the benefits they derive from slave labor…The idea of enforcing the laws, has but one object, and that is collection of the taxes, raised by slave labor to swell the fund necessary to meet their heavy appropriations. The spoils is what they are after – though they come from the labor of the slave.”
Of course others in the South such as Jefferson Davis and Robert E. Lee disagreed with Stephens that secession and the war were about slavery"
Beginning in late 1862, James Phelan, Joseph Bradford, and Reuben Davis wrote to Jefferson Davis to express concern that some opponents were claiming the war "was for the defense of the institution of slavery" (Cooper, Jefferson Davis, American, pp. 479-480, 765). They called those who were making this claim "demagogues." Cooper notes that when two Northerners visited Jefferson Davis during the war, Davis insisted "the Confederates were not battling for slavery" and that "slavery had never been the key issue" (Jefferson Davis, American, p. 524).
Precious few textbooks mention the fact that by 1864 key Confederate leaders, including Jefferson Davis, were prepared to abolish slavery. As early as 1862 some Confederate leaders supported various forms of emancipation. In 1864 Jefferson Davis officially recommended that slaves who performed faithful service in non-combat positions in the Confederate army should be freed. Robert E. Lee and many other Confederate generals favored emancipating slaves who served in the Confederate army. In fact, Lee had long favored the abolition of slavery and had called the institution a "moral and political evil" years before the war (Recollections and Letters of Robert E. Lee, New York: Barnes and Noble Books, 2003, reprint, pp. 231-232).
"The people of the Southern States, whose almost exclusive occupation was agriculture, early perceived a tendency in the Northern States to render the common government subservient to their own purposes by imposing burdens on commerce as a protection to their manufacturing and shipping interests. Long and angry controversies grew out of these attempts, often successful, to benefit one section of the country at the expense of the other. And the danger of disruption arising from this cause was enhanced by the fact that the Northern population was increasing, by immigration and other causes, in a greater ratio than the population of the South. By degrees, as the Northern States gained preponderance in the National Congress, self-interest taught their people to yield ready assent to any plausible advocacy of their right as a majority to govern the minority without control." Jefferson Davis Address to the Confederate Congress April 29, 1861
Major General Patrick Cleburne was another who disagreed with Stephens.
The conqueror's policy is to divide the conquered into factions and stir up animosity among them...It is said slavery is all we are fighting for, and if we give it up we give up all. Even if this were true, which we deny, slavery is not all our enemies are fighting for. It is merely the pretense to establish sectional superiority and a more centralized form of government, and to deprive us of our rights and liberties." -General Patrick Cleburne
The Newspapers of the largest ports in the original 7 seceding states also disagreed that it was "about" slavery.
"The real causes of dissatisfaction in the South with the North, are in the unjust taxation and expenditure of the taxes by the Government of the United States, and in the revolution the North has effected in this government from a confederated republic, to a national sectional despotism." Charleston Mercury 2 days before the November 1860 election
"They [the South] know that it is their import trade that draws from the people's pockets sixty to seventy millions of dollars per annum, in the shape of duties, to be expended mainly in the North, and in the protection and encouragement of Northern interests. These are the reasons why these people do not wish the South to secede from the Union. They, the North, are enraged at the prospect of being despoiled of the rich feast upon which they have so long fed and fattened, and which they were just getting ready to enjoy with still greater gout and gusto. They are mad as hornets because the prize slips them just as they are ready to grasp it. These are the reasons why these people [the North] do not wish the South to secede from the Union." The New Orleans Daily Crescent 21 January 1861
the Klan was another matter. That arose during the Occupation in response to the disenfranchisement of the vast majority of the voters in the Southern states, the massive corruption and theft of property by the occupation governments and by terrorism inflicted upon the citizens when they objected by the Union League.
The Ku Klux Klan was created to terrorize the ex-slaves out of participating in this political plundering racket operated by the Republican Party. The Republicans kept promising to share the property of White Southerners with the ex-slaves, which of course they never did and never intended to do. Had the Republicans not used their victory and their monopoly of political power to line the pockets of the thousands of political hacks and hangers on who were the backbone of the party (the "carpetbaggers") the Ku Klux Klan would never have existed. This in fact was the conclusion of the minority report of an 1870 congressional commission that investigated the Klan. "Had there been no wanton oppression in the South," the congressmen wrote, "there would have been no Ku Kluxism" (Congressman Fernando Wood, "Alleged Ku Klux Outrages" published by the Congressional Globe Printing Office, 1871, p. 5). The report continued that when Southern Whites saw that "what little they had saved from the ravages of war was being confiscated by taxation . . . many of them took the law into their own hands and did deeds of violence . . . . history shows that bad government will make bad citizens."
Irrelevant. Focusing on their alleged reasons for wanting to leave is dismissing their right to leave. Their reasons for leaving may have been morally wrong or even stupid, but you cannot gainsay someone's right to do something because you don't like why they want to do it.
And that being said, Paul Craig Roberts argues their claims of wanting to leave over slavery were simply intended to be a clever legal strategy for getting out of the contract that was the US Constitution. By arguing that the Northern states had breached the contract first, it gives them the legal right to revoke the contract. In other words, all the statements claiming this was the reason were just a ploy.
https://www.paulcraigroberts.org/2018/11/13/a-civil-war-lesson-for-the-uneducated/
With the US Congress and the Northern states willing to vote to pass a slavery forever amendment, the fact that the Southern states didn't accept this deal implies that Paul Craig Roberts is correct. They weren't after protection for slavery, they were after the money and used "slavery" as an excuse to get out of the contract.