You really don't seem to be willing to consider that two reasonable people can have differing opinions about the same situation. Who appoints these "reasonable outside observers"? And who defines who or what is "corrupt"? It all just boils down to what you yourself view as corrupt or reasonable.
Also, if Spain is like most countries (but I really don't know this for a fact), but elected representatives will have signed or sworn or something similar to uphold the constitution.
I think the legitimacy difference has to do with inalienable rights. People have a right to representation in their own governance. That's what the dictator gets wrong. People also have a right to liberty - i.e. To not be slaves. A "law" that says you can't ever secede is making people slaves by taking away their right to withdraw from the political union they're a part of.
The argument above is that it's legal because it's written down. What I hoped to call attention to is that simply writing something down and calling it a law isn't necessarily better than what a dictator does.
So...people's personal opinions then?