The Star Wars universe is a crazy place, but there are consistencies that you can count on now and again. One of those is that whenever you run into a Force-wielder who turns to the dark side, you can usually count on them wielding a red-bladed lightsaber. The movies have never explained why this is, and fans have just grown accustomed to seeing the bad guys use crimson energy. Now we finally have an in-canon answer to why this is the case thanks to the Star Wars novel Ahsoka: because they're naturally incapable of using any other color.
The latest Star Wars novel follows Ahsoka Tano, Anakin Skywalker's apprentice from Star Wars: The Clone Wars, during the time period between her last appearance on that series and her return in Star Wars Rebels. In the midst of her adventures, it is explained (via ScreenRant) what the deal is with the red lightsabers. For the causal Star Wars fans out there, lightsabers channel their energy though a kyber crystal, which a Jedi find on the planet Ilum or several other locations. However, it's a more complicated process than just snatching a random one and putting it in the lightsaber. A Jedi must use the Force to guide them through the selection process, and once a connection is formed, that's when the crystal turns blue, green or another color (like purple).
Conversely, a dark side user, whether they're Sith or a Knight of Ren, doesn't have that same connection. They can only obtain one of these crystals by either stealing them or taking them from a slain enemy, and then they have to bend the crystal to their will. This leads the crystal to "bleed," hence why the villains always have red lightsabers. The corruption means those other colors are inaccessible. Some of you will wisely point out that Anakin Skywalker slaughtered younglings and Separatist leaders with a blue lightsaber in Revenge of the Sith, but that was his old blade. He hadn't gained his Sith weaponry yet.
Although the Star Wars franchise had built an Expanded Universe across novels, comics, games and more since shortly after A New Hope, Lucasfilm declared all material outside of the movies and Star Wars: The Clone Wars TV series non-canon in 2014 so that they could have a fresh start under the Disney regime. Starting with A New Dawn, all Star Wars novels are canon, which means this red lightsaber explanation counts towards to movies as well, even though there's no guarantee it will ever be outright stated on the big screen. There may never be a reason to do so, as in the end, the red lightsabers are simply another way to distinguish who the Force-wielding villains are in case the black clothing and sinister dialogue don't make it clear. This fact from Ahsoka is mainly for the hardcore Star Wars fans who relish absorbing encyclopedic knowledge from a galaxy far, far away.