The country had been dominated by the south for most of the prior 80 years and yet the North saw no need for rebellion or 'bloody fratricidal war'. Why did the south see the need for this if not for the purpose of protecting their slavery?
Seen it, been there, done that. During the 80 years of supposed Southern domination, what were the abuses of the federal government? How many Southerners called for overturning the constitutional guarantees of the North? Undoubtedly, there were abuses of government, but I'm not sure a great deal of them rested on sectional hostility, save those surrounding the war of 1812.
Why did the south see the need for this if not for the purpose of protecting their slavery?
Of course they were leaving to protect their slavery, but it was framed in the much larger picture of objective rule. For years before the war an outright state of hostility existed in Kansas & Mo. Thousands of Northern editorialists, and preachers open mourned the death of the terrorist John Brown. Any victory which returned the country to the Constitutional agreements on slavery - Dred Scott, the FSA, etc was bemoaned throughout the North as an opressive injustice inflicted by Southerners, and now there was an administration and sufficient congressional support to wrest the government from it's constitutional chains (and they made clear their intent to do so).