Yeah, give that a rest for a moment. Ex Post Facto is referring to "After the Fact", as in making something a law, (Or suddenly morally outrageous) "after the fact."
Before the war? Slavery was A-OK with the Union. After the war? Slavery was an abomination and justified the deaths of 600,000 men and the devastation of vast swaths of populated areas.
Pushing this argument actually makes good financial sense. By declaring it an abomination, they didn't have to pay anyone for what they took away. Billions of dollars in financial assets just evaporated as punishment because the Southern states fought back.
The FedZilla didn't have to pay any claims, because they retroactively made a previously legal thing into an illegal thing. Now that worked out just swell for them, didn't it?
Well, maybe your heroes shouldn’t have opened up fire on a federal installation and launched an aggressive war against their own legitimate government, because they feared that a Republican president might slightly impinge on their beloved “right” to enslave other men, for their own financial, material gain. Then all that bad stuff wouldn’t have happened to them.