Are implying that, for example, if Catalonia, Bask or Scotland become a country situation will be ripe for killing Spanish or English man living there?
This is essentially what had to be dismantled in Northern Ireland - it was under the control of one "group" usually defined as Protestant. Transferring it to a situation under human rights law where all religions could live with equal protection under the law (including protection from the state and police by means of human rights law) was critical to ending the conflict.
None of the Catalan, Basque (this is the usual English spelling, are you perhaps thinking of the Russian Басков?) or Scottish independence movements have ethnic cleansing ambitions, although these are usually attributed to them by their enemies.
Construction and definition of a nation state is a complex issue not reducible to glib remarks about "liberty".
If one group have ethnic cleansing ambitions they will do it anyway. This whole "equal protection and human rights" rhetoric is laughable where on group seriously oppresses other. It is better off that all groups live in more autonomy and if one is not happy, moving to another place is better than living under threat. Of course, this is not the "liberty" people are longing for, but it is the preferable choice.
"The SNP believe that immigration is essential to the strength of our economy and adds greatly to our cultural fabric."
https://www.snp.org/pb_what_is_the_snp_s_policy_on_immigrati...
The SNP is sometimes described as being about civic nationalism rather than ethnic nationalism:
https://en.wikipedia.org/wiki/Civic_nationalism
Edit: Arguably the closest we've got to ethnic cleansing was the Highland Clearances - which were inflicted on Gaels by landowners.