Hey, I don't think we were necessarily justified.
However, it's not the same case. North and South entered into a contract. Both were represented in Congress (thanks to the three-fifths compromise, the South generally dominated). It was not a "loose" confederation of states---that didn't work, and the Constitution was born. The Southern states were not shut out of the government until after their rebellion and defeat. Any secession should have gone through Congress---states can't just unilaterally pull out like South Carolina did and seize federal property. Can you imagine Illinois seceding because Bush was (sorta) elected president in 2000? Preposterous.
In the case of the revolution, our ancestors were subjects
of the British throne, if a subject has a right to rebel against his lord and king how much more should this right be attributed to free citizens
? If we are to believe in the founding principles of this republic, in the ideal of government by consent of the governed; then we cannot be consistent and deny any group of people the right to abolish their government and form their own when they determine this to be necessary.
If Illinois were to secede for any reason, including the election of a disliked president, I would strongly support their right to do so. I'd probably wish them a good riddance but would certainly support their right, quite possibly by supporting them with force of arms, to secede if the citizens of their state determined this to be consonant with their liberty, security, or happiness. A people are not required to gain the consent of a despot to exercise their inalienable right to self-determination.