Of course, one could point out that it was rather the Confederacy's aim to "Form [a] Government...destructive of these ends...pursuing invariably...to reduce them under absolute Despotism". After all, one side wanted to retain human bondage, the other side was even at the start less married to it and ended by abolishing it. As written the Declaration of Independence is not a blank check to secessionism, and you can certainly make an argument that the South's secession was not in any way justified by the arguments given by the Declaration given its purpose and goals.It's right there in the Declaration of Independence:
In any case, arguing about whether the South seceding was one of the ten worst decisions in human history seems to me to be missing the forest for the trees; the actual worst decision (or, well, one of them), without which the whole issue would have been moot, was establishing forced labor in the first place. Of course this didn't occur just one time, but I think it's fair to say that the African slave trade alone would make forced labor one of the single biggest causes of suffering and death in human history, which as per my own argument earlier makes the establishment of such forced labor an objectively terrible decision, whatever the short-term justifications.