South Carolina was the first slave-holding state to secede from the Union December 20, 1860. Why did South Carolina secede?
Read the full declaration of causes for why SC seceded from Wikipedia.
One can read the complete declaration and sufficiently conclude that South Carolina seceded over what her politicians felt was the right to maintain slavery as an institution.
I’ve excerpted out the statements related to slavery.
DECLARATION OF THE IMMEDIATE CAUSES WHICH INDUCE AND JUSTIFY THE SECESSION OF SOUTH CAROLINA FROM THE FEDERAL UNION.
But an increasing hostility on the part of the non-slaveholding States to the Institution of Slavery has led to a disregard of their obligations, and the laws of the General Government have ceased to effect the objects of the Constitution.
….the current of anti-slavery feeling has led her (New York) more recently to enact laws which render inoperative the remedies provided by her own law and by the laws of Congress….
The right of property in slaves was recognized by giving to free persons distinct political rights, by giving them the right to represent, and burdening them with direct taxes for three-fifths of their slaves; by authorizing the importation of slaves for twenty years; and by stipulating for the rendition of fugitives from labor.
For twenty-five years this agitation has been steadily increasing, until it has now secured to its aid the power of the Common Government. Observing the forms of the Constitution, a sectional party has found within that article establishing the Executive Department, the means of subverting the Constitution itself. A geographical line has been drawn across the Union, and all the States north of that line have united in the election of a man to the high office of President of the United States whose opinions and purposes are hostile to slavery. He is to be entrusted with the administration of the Common Government, because he has declared that that “Government cannot endure permanently half slave, half free,” and that the public mind must rest in the belief that Slavery is in the course of ultimate extinction.
Harper’s Weekly, November 23, 1861
In Georgetown, Charleston, Colleton, and Beaufort Districts are several parishes, in which more than 90 per cent of the inhabitants are slaves. In Lower All-Saints parish, Georgetown District, the population is reported to be 222 whites, no free negroes, and 6468 slaves. In the parish of St. Luke, lying west of Broad River, in Beaufort District, the population is 88 slaves to 12 whites. In the city of Charleston the colored population was 53 per cent., six-sevenths of which were free colored people. Only 53 per cent. in the city reduces the percentage of the whole District to 65 per cent., although in the parishes of St. James, Santee, St. Thomas, St. Andrews, and St. Johns, which lie toward the coast, more than 90 per cent. of the inhabitants are slaves.