One of the primary planks of the emerging Republican Party of the 1850s was preventing the extension of slavery to the emerging western territories. Most of those in the Democratic Party in the free states believed that slavery should be determined by popular sovereignty. In other words, when territories acquired a large enough population and petitioned Congress to become as state, those citizens could then vote by popular referendum whether to come into the Union as a free or slave state and encode such laws in that state's constitution. Most Democrats, whether North or South believed that geography and climate would ultimately determine if slavery would be feasible for a new state, but they wanted the option.
Republicans, regardless of their political bent (conservative or radical on the slavery issue) believed that if slavery was not allowed to expand it would die. Southern Democrats viewed the issue likewise. Like a tree that has its roots narrowly confined, it will eventually wither and die over time. Or, another even more popular analogy of the period was that shown in the political cartoon above: A scorpion, when surrounded by a ring of fire, would chose to sting itself to death rather than be consumed by the flames.
Southerners, particular the planter politicians--but many yeoman, too, who benefited from planter gratuities and hoped to elevate themselves socioeconomically one day to become slave owners--believed that slavery must expand in order to survive. They fully understood how slavery benefited their section. If slavery died, so would the Southern way of life. Politically, economically, socially, the South would change forever. Planter politician extraordinaire, John C. Calhoun, had argued during the debates over the Compromise of 1850--half a decade before the emergence of the Republican Party--that, "the Southern section regards the relation [between slaver and master] as one which can not be destroyed without subjecting the two races to the greatest calamity, and the [Southern] section to poverty, desolation, and wretchedness; and accordingly they feel bound by every consideration of interest and safety to defend it." And attempt to defend it they did.
Why did Southerners threaten, beat, and banish abolitionists and anti-slavery proponents that they found within their borders and in their communities during the antebellum years? Because, those that espoused such ideas were viewed as a deadly threat to that cherished way of life that they believed the Constitution entitled them to. Why did Southerners seemingly in haste begin the domino process of secession when Republican Party candidate Abraham Lincoln was elected president in November 1860? For very much the same reason. Although Lincoln was no abolitionist--at least not in 1860--he was a Republican and his party had determined to limit slavery to those states where it currently existed and let it expand no more. That was a direct threat and challenge that Southern honor would not tolerate. After all, their forefathers had fought in the Revolutionary War, just as vitally as Northerners' ancestors had to establish an independent nation and create a form of government which they believed guaranteed their domestic institutions if they so chose them.
Compromise was at an end. And then war came, and those that sowed the wind, reaped the whirlwind.