Ghildrean: Catalans did a non-binding vote in 2014, and even then it was illegal but the goverment did not interfere, and they voted YES. They have been trying to negotiate with the Spanish goverment since then, but both sides have not made any progress (the Spanish goverment because they didn't want to, and the others because they wanted to do it fast). After a while, the Catalonian parlament has decided they want no more of this bullshit, they broke every democratic procedure at the start of September to force, without any consent, this voting process and declaring it will be a binding vote.
Basically, they wanted people to vote no matter what, and since Europe also told them that it was illegal, they resorted to do the opposite, show how bad the Spanish goverment was to the world. The results of the voting process don't matter (well, the Catalonian president has declared that they will start the independence process even if only 42% of the poblation voted and 90% of that percentage voted YES), the important thing is how violent, repressive and dictatorial the Spanish goverment is. Hell, a lot of people voted MULTIPLE times since there was no way of verifying who already voted and some of the ballot boxes were already filled before the start of the voting process.
EDIT: As it has been pointed out later, the results of 2014 were an outstanding 80% yes for independence. I stand corrected and half of the message was meaningless. I have modified the message, although I should delete it entirely.
Do you know why the referendum is illegal? It is illegal because the Spanish Constitution states that Spain cannot be broken up. So under the Spanish Constitution it would never be legal for any part of Spain to leave. Does that seem right that a part of a country cannot, in any circumstances whatsoever, declare independence?
I personally believe that distinct regions should have the right to self-govern, particularly regions where their role in the part of other countries is historically quite new.
If there are concerns with the voting process, those can be dealt with, for example by having international independent observers come in, but it is not a reason to say that there cannot be a vote in any circumstances whatsoever.