I thought this analysis article was really thought-provoking and now I have uncertain and mixed feelings about the referendum: https://www.nytimes.com/2017/09/29/world/europe/independence... . Basically, when it comes to things like this there's a conflict between two international ideals: integrity of national borders and human rights . And there are some thought provoking ideas in the article. If you think about it…
There's more nuance when we're talking about entire states, but I believe the same principle should apply. This particular vote was irregular enough to not really be legitimate, but the Spanish government should recognize that some sort of Catalan referendum has a right to take place.