But professorTuring did not say a cause, he said "Catalonian cause".
You are right in that promoting criminal activities can help a cause in very specific cases, but I don't think is relevant here. Gandhi, MLK, etc had legitimate reasons to break the law. Unless the Catalan independentist movement can convince the international community that they also have a legitimate reason, taking the law in their hands will not help them with their objective.
By the way, the ruling government has never called the independence movement criminal. It's perfectly legal to support independence, demonstrate, create, fund and promote independentist political parties, try to change laws to achieve self-determination... All that is perfectly legal in Spain. Disobeying court orders or mishandling public funds, however, it is not legal in Spain or any other country I know.
You ask for a different approach to make a case against Catalonian independence, but I do not think this is fair. The burden of proof is not on those who think they should not break the law, but in those who think they should. If you want to form an opinion by yourself I suggest you to try to find the reasons they have to want independence, from their own sources, but crosschecking any "historical facts" you find. If after that you think they have a legitimate reason to break the law, I'd like to hear it.