The fact of the matter is Southerners were responding to an imaginary threat to themselves. The South cared about states rights insomuch as slavery was being sustained through federalism; the South had moved to infringe on the North only a few years earlier with fugitive slave legislation. I agree it can be viewed as irrelevant what came about from the war, or even why they want to war, but I don't see Lincoln's actions as illegal.
It's a gray area because the constitution never explicitly addresses secession but does refer to rebellion and grants the president such powers as Lincoln sought when rebellion occurs.
It wasn't even a revolt by the people. It was a revolt by the legislating class.