"settlers" is the perfect word to describe their social role. Far better than "Nazi" or "fascist" which ditch precision for emotional weight.

They are behaving as genocidal maniacs because of their social role within settler colonialism, not because each person believes in fascism or because they're just behaving "fascistly" or whatever.

They may be indistinguishable at times, but the word "settlers" paints quite an accurate picture of what is happening: Manifest Destiny or Lebensraum.