This question already has an answer here:
In the original trilogy (A New Hope, Empire Strikes Back, Return Of The Jedi) calling Luke, Leia, Hans, etc made sense. They're rebelling against an evil empire.
In the latest set of Star Wars movies, they are still called The Resistance (and generally see themselves as such), and the First Order routinely calls them "Rebel scum".
Except if my understanding of the plot is correct The First Order is actually the rebel scum! They destroyed the capital, then used their fleet of ships to began taking control of the galaxy in between The Force Awakens and The Last Jedi.
It seems like it would make more sense for the First Order to use "democratic scum", "government scum". Even during a civil or revolutionary war, the insurgents don't call the other side rebels - they call them Brits, Northerners, etc, not rebels.
Has this ever been addressed either in Star Wars media or in cast and crew interview?
EDIT: Out of universe calling people rebel scum is part of Star Wars. There is a reason many of the walk-ways in Star Wars don't have guard rails - I wondered if this is the same.