Lightsabers come in multiple colors, either based on the Jedi’s chosen profession or turned that way through the force (geddit?) of their personality. But Sith lightsabers are always red. It seems there’s a reason for it.
According to the new comic book Darth Vader #1, written by Charles Soule, it’s because the Sith torture the crystals.
It begins with taking a crystal from a Jedi’s saber. Although it isn’t explained, it’s implied that a Sith would have to kill the Jedi first, as the only way to take a saber off of a Jedi is over their dead body.
Then comes the painful part. The Sith channel their rage into the living crystals. As Palpatine explains,
"The crystal from any Jedi's saber will do. As you know, the Kybers are alive, in their way. Like any living thing, they can feel pain. Through the dark side, you must pour your pain into the crystal. And when, at last, the agony becomes more than it can stand ... a beautiful crimson. The color of your rage."
We knew that the Sith were evil. We didn’t know they were that evil.
This is yet another way that the new Disney-owned Star Wars universe is differentiating itself from the old one. Prior to this, according to Star Wars’ wiki, fans knew that the Sith created their own synthetic crystals to power their sabers.
Star Wars fans first learned more about the mystic crystals in the 1978 novel Splinter of the Mind’s Eye, by Alan Dean Foster. It was the first of many, many Star Wars Expanded Universe novels, which are now known as Star Wars Legends (and are now non-canon).