The Democrats were as wrong as they are today.
And there is no right to destroy the Union because you fear your slaves will be taken from them, none.
The Union of the Crowns (United Kingdom) was not "destroyed" when 13 colonies left it, and the Union of the Colonies would not have been destroyed either. To claim it would have been is just hyperventilating.
Secondly, the right to leave the Union (as articulated in the Declaration of Independence) is not conditional. It doesn't matter what someone's reasons for leaving are, they have the right to do so regardless of their reasons.
You are not the moral police, you don't have some God given moral authority to decide that rights are only valid if they meet with your moral approval.
The only thing required for them to exercise the right of independence was the belief that the existing government no longer served their interests. They so decided in orderly elections, and they chose to leave.
The rest is just screeching from the people who were losing the money that came from their export production.