The best answer to your question is to read each states decoration of succession. To me doing so revealed that slavery was by and large the biggest issue.
Constitutional or not, no man has any "right" to own another. I don't care what any written document has to say about it. Given the option between letting it die out in a few decades, or a bloody civil war, I think a bloody civil war is the better option.
It’s interesting the parallels between slavery then and gun ownership now. There’s a lot of folks that think the same of gun owners now as they do of slavery owners back then, up to and including your opinion of a written document.
The War Between The States has nothing to do with freeing the slaves. If that were the motivation to invade the South then why not set the example and free those slaves owned in the North? Because those behind the war didn’t really care about the plight of the African Slave, it was simply a power grab in which over 600,000 Americans died.