Red is a symbol for hostility, susceptibility, and wickedness. Following are some of the reasons for the red color of Sith lightsabers.

A possible reason is that the Sith create their lightsabers in a unique fashion, forcing the appearance of blood-spewing out of the blade by forcing all the negative and evil sentiments into the crystals. This gives it the appearance of being red.
Since they couldn’t manipulate and ignite Kyber crystals, the Sith were forced to create synthetic kybers. They were unable to form a relationship with the Sith lightsaber kyber crystals as the Jedi could, which was the main cause. Their artificial kybers were red, therefore the lightsabers also eventually came out to be red. As a consequence, we may see Kylo Ren, Count Dooku, Darth Vader, as well as Darth Maul wielding the red-bladed Sith lightsaber.
It’s a tradition-based task. Despite having access to numerous hues, the Sith choose red as a symbol of their depravity. Some claimed that these kyber crystals were taken from Jedis and “mistreated” until they were tainted and glow crimson, reflecting the hatred and enmity of the Sith.
Each of the hues of the lightsabers has a significant connection to the user’s individuality. The Sith utilised red lightsabers as they filled the crystal with all of their dark energy to make it work. Their adherence to the evil side and mutual support are represented by a crimson lightsaber.
Red lightsabers are used by the Sith to represent their haste, desire for power, extreme vanity, and degrees of selfishness. Red Sith lightsabers are symbolic of their bravery and resistance to restraint. Additionally, it displays their recklessness and fearlessness.