Is about a trio of like 'judges' that travel around the galaxy and 'exterminate' civilizations that have become too evil.
And basically there is a humanoid species that had developed a horrifying type of 'communism', and those 'judges' destroy the planet.
Volcanoes and slavery are mentioned.

No further details because of spoilers and it should already be easily recognizable.

If anyone here has read that short story, or knows the title or author, then please do tell.