Well, we have a user called /u/secessionisillegal, who wrote a long and quite detailed answer which directly deals with this issue: If slavery was protected by the constitution via Dredd Scott (1857), and a constitutional amendment requires 75% of States and congress to approve, then how was secession the best response to Lincoln’s plan to ban slavery in the western territories? Lincoln didn’t have the votes.

This is also an issue that /u/freedmenspatrol deals with quite expertly in this answer: Why can't the Civil War be called the "War of Northern Aggression," or a war about states' rights?