The “all the bad guys- and only the bad guys- have red lightsabers” is a prequelism I can do without.
It’s not a prequelism.
Yes, it is. Or are there other darksiders in the OT with red lightsabers aside from Vader I’m unaware of?
Just because there weren’t any others doesn’t change the fact that it was used to portray evil and to contrast with the good, portrayed by blue. If there was another evil lightsaber-wielder in the OT, he would have had a red blade to keep the feel. A real prequelism is giving all darkside force users the name “Darth” as a title, rather than it being Vader’s first name.
The reason Darth had a red saber in Star Wars was to create the simple contrast of good and evil, just like everything else in the film.
That’s an out of universe reason. There’s no reason to think it’s an in-universe reason going off the OT alone.
Why does it matter if there’s a specific in-universe explanation for it? It’s a specific aesthetic that was used to portray something in the film(s). I never said there was an in-universe reason, I just don’t think something such as this needs one.