It seems obviously wrong for the Spanish Government to suppress a Catalan referendum. Here's some factors to consider.
The Catalan nationalists (maybe about 40% of the population) appear to believe they are justified for two reasons. One is their referendum in which 90% voted for independence, but many unionists naturally didn't think it right to vote in an illegal referendum and 90% of the 42% turnout is only 38% of Catalan voters. So no justification there.
What about the fact that so many Catalan MPs are nationalist?
From Guardian newspaper, UK...
It is of the utmost importance to emphasise that Catalonia already has a clear mandate for independence, achieved in the parliamentary elections on 27 September 2015. On that occasion, around 75% of Catalan voters had their say in the most participative elections ever in Catalonia, in which the pro-independence parties came out with an absolute majority in parliament: 48% of voters supported pro-independence parties, whereas 39% opted for unionist ones. The remaining 13% voted for lists that did not align themselves with either the “yes”or “no” campaigns. Unquote.
But in Scotland in 2015, people elected 56 out of 59 MPs who were nationalists but that didn't indicate a majority in favour of Scottish independence, or even close to that. In the 2014 referendum, the vote was 55% for they Union and 45% for Independence - and there's evidence the Unionist vote may have strengthened since then. So electing a majority of nationalist MPs does not necessarily indicate a wish for independence.
In the 1970s about 90% of Catalans voted for union with Spain and for the current constitution which makes an independent Catalan by referendum very difficult.
A majority of Kurds actually do want independence. Taiwan would like to be recognised as a State. Many Tibetans favour an independent Tibet. The Falkland Islanders and the Gibraltarians emphatically wish to stay British.
There is the question of timing, obviously, but when is self-determination not a right? Many think Humanist organisations should not take a position on such a political 'hot potato'. It is an ethical question and a political one.