Slavery wasn't at the forefront until Lincoln attempted to free Southern Slaves, specifically, as a war measure to further cripple the southern forces. The secession was because of several things, one of the biggest being John Adam's Tariff of Abominations, which caused the South's economy to decline further. Other reasons include an expanding government, as the south believed, rightfully, that Federal Law shouldn't trump State Laws, as the States were originally intended to legislate for themselves, and to be able to opt out of the Federal Government's Nation-wide laws. Lincoln's wanting to free the slaves fell under this category because it's an example of the government eroding state rights through its expanding power.
So, in other words, it had been coming for quite some time, and was about waning agriculture vs expanding industry, and the rapidly expanding government becoming more powerful than the states, due to Presidents, like Lincoln, stomping all over State rights. The US will likely never recover from Presidents like Woodrow Wilson, FDR, and Lincoln. The South just wanted to opt out.
I don't know why you guys want it to be about slaves, the south was leftist anyway, seems like you're attempting to further ruin your own image.