"Slavery is founded on the selfishness of man's nature-opposition to it on his love of justice. These principles are in eternal antagonism; and when brought into collision so fiercely as slavery extension brings them, shocks and throes and convulsions must ceaselessly follow."
Abraham Lincoln, 1854
There are a lot of the things about the rebellion that are offensive to the ideals of America, but the motivation for the secession is particularly grotesque. They'd speak noble words about the liberty of man, but their whole motivation was their own selfish aims which were based on the non-liberty of their fellow man. Our American Revolution showed that moral right will endure and triumph over material superiority. Conversely, the quick demise of the rebellion shows that morally flawed causes cannot endure.
The short lived timeframe of the 'Confederate' rebellion demonstrates a foundation constructed on evil can not stand, nor shall be resurrected in any respect in today's America.
"Whenever I hear any one arguing for slavery I feel a strong impulse to see it tried on him personally."
Abraham Lincoln, March 17th, 1865, Speech to 140th Indiana Regiment. Lincoln, Abraham, 1809-1865. Collected Works of Abraham Lincoln. Volume 8.