Bleeding Kansas demonstrated the level of extremism of some of the Northern abolishionist. That they were willing to murder southern settlers for no other reason than they were southerns and southerners were PROBABLY slave owners or supported slavery.
The Harpers Ferry raid in 1859 show that these same abolishionist were still willing to incite open rebellion for the whole nation to accomplish their purposes by creating a slave rebellion. An abortive attempt that had no chance of succeeding in reality but did succeed in placing the notion in the southerners minds throughout the south of a slave rebellion. And althought John Brown was hanged for his crime the north generally takes up his cause and makes a martyr of him. "John Brown's body lies a molding in the grave" but what goes marching on? This tainted the 1860 election of Lincoln for the southerners.
And final the United States Color Troops and reconstruction. The United States made enemies of the blacks to the southern people by making them soldiers. The south saw the war as a "White Mans War" fought over principals. The south used the "blacks" in service to free up the "white" man to serve in the Battle lines, while the north placed the "black" man in the Battleline to save the "white" man. These U.S. Color Troops were later used to enforce the reconstruction rule over the disenfranchised southern people in the role of conquerers.
These things did more to divide the southern blacks and whites into the segragated communities that they became, more than slavery ever did.