“Corrupt the kyber crystal. Teach it your pain. Teach it your anger. Hear it sing a hymn of darkness. Make it bleed.“
Some people have wondered why the lightsabers of dark side practitioners were red. In Legends, it was because the crystals were synthetic. However, the new Canon provides a far more satisfying explanation. The minerals that power lightsabers, kyber crystals, are living things. In order to submit to the will of a dark side practitioner, the crystal must suffer and bleed. To achieve this, the Force wielder would imprint their own dark side rage and hatred into the crystal. When the crystal and the blade it generates becomes red, it effectively becomes a slave to the will of the dark side practitioner. So far, there are three prominent examples of this ritual.
After claiming the lightsaber of a Jedi he killed, Darth Vader returned to Mustafar to perform the ritual. Initially, Vader’s efforts backfired and the crystal rejected him because his dark side rage was no strong enough to make the kyber crystal submit. However, after renewing his resolve and commitment to the dark side, Vader was able to make his kyber crystal suffer and bleed. The power Vader summoned was so intense that it ravaged Mustafar with earthquakes and devastating storms.
While preparing his Final Order fleet of Star Destroyers on Exegol, Palpatine harvested a mountain of kyber crystals to power the planet-killing cannons of his fleet. He had his Sith Eternal cultists working around the clock to make the mountain of crystals submit to his will. The hellish pain that the crystals were enduring was so intense that you can hear the crystals scream like a damned soul. The dark side energy the bleeding crystals were producing was so powerful that it completely overwhelmed someone of Darth Vader’s caliber. Vader even screamed like he never screamed before as he was bombarded by the crystal’s power. It is possible that Palpatine was feeding off of the crystal’s pain since he was completely unaffected by its energy. According to Palpatine, the dark side energy his mountain of kyber crystals were producing was unlimited power that was strong enough to burn his target’s flesh and mind, ready to incinerate them in an instant.
After his fall to the dark side, Kylo Ren performed this ritual on the kyber crystal of his Jedi lightsaber. Kylo Ren imprinted his rage and hatred so intensely that the crystal cracked in his hand. In addition, the ritual produced a pulse of dark side power that rippled through the Knights of Ren’s ship. Because his crystal cracked, Kylo Ren’s lightsaber was unstable and crackled, which threatened to make the lightsaber explode. To counter this, Kylo Ren remodeled his lightsaber so it would have its cross guard. The cross guard acted as filters that vented the excess energy produced by the cracked kyber crystal.
Overall, I like this explanation for red lightsabers better than the one in Legends. It provided a much deeper look into the philosophy of the dark side. It also symbolizes the cementation of a dark side practitioner’s commitment to the dark side.