I would've supported the Union, but in a more limited war. The Confederacy attacked the Union. As despicable and disgusting as its motives were and the regime was, I do believe the Confederacy had a right to secede. It did not, however, have a right to attack the remaining United States of America. The Union had every right to defend its sovereign territory.
Insofar as the post-war aftermath goes, I would have been firmly in the Radical Republican camp.
Why? Why do people believe that a bunch of butthurt aristocrats had the right to try and quit the country?
Why do people believe that they did not? The Constitution is silent on the issue and trying to claim support from the Articles on the idea of a perpetual union is preposterous given that the method of ratification of the Constitution was totally illegal under the Articles.
If only Governor Pickens hadn't been such a penny-pinching idiot there would have been no Fort Sumter. (The Union garrison in Charleston had originally been located in the indefensible Fort Moultrie. Pickens could have had South Carolina troops occupy the unfinished Fort Sumter before Major Anderson moved his troops, but he declined because he wanted the workmen there to stay on the Federal payroll for the moment.)