The Sith in Star Wars are easily identifiable as the villains because of their nightmarish appearance. Their actions are evil and self-aggrandizing, but their demonic appearance is what allows the Sith to dominate every scene they appear in.
Lucas wanted his audience to easily know that who the villains were, so he incorporated many visual cues that represent the Sith. Some of those signals are demonic tattoos, animalistic eyes, dark clothing, masks and machines.
The reason that the Sith are so nightmarish looking is that George Lucas wanted them to be easily recognizable. Lucas is quoted in a Sunday Times Magazine story called “Master of the Universe” where he says, “Someone has to tell young people what we think is a good person. … You need that in a society.” Part of Lucas’s intent in creating Star Wars is to communicate a mythology that defines right from wrong and educates society about morality.
In real life, looking a certain way definitely does not mean anything about someone’s character, but in a story – and especially in mythology – visual cues are important. If Lucas wanted his audience to learn from his galaxy far, far away, he needed the heroes and the villains to be easily recognizable. That is why the Jedi always dress the same and why the Sith look like something out of a nightmare.
The script for The Phantom Menace actually described Maul as “a vision from your worst nightmare,” and his appearance did not disappoint. When he first appeared onscreen in 1999, he was clearly the depiction of something demonic. His blood-red skin and black tattoos inspire terror and an automatic knowledge that he is unnatural and something to be feared. His tattoos are even rumored to be Sith in origin.
Asajj Ventress is also known for her haunting appearance. Ventress is lithe and acrobatic in her fighting style, but even before she draws her curved lightsabers, she gives off an air of wickedness. Like Maul, Ventress has tattoos on her head. Each of the twelve markings represents one of the twelve warlords she killed as a child. They had killed her Jedi master, and Ventress had wanted to remember her revenge, so she permanently marked her skin.
Darth Sidious is best known for his wrinkled and decrepit face as the Emperor, but even before becoming the victim of his own Force lighting, Sidious embodied evil in his appearance. In the James Luceno’s Darth Plagueis novel, a young Darth Sidious once saw himself in a mirror as he used his lightsaber in a murderous rage: “Sidious caught a glimpse of himself in an ornate mirror; face contorted in rage, red hair in electrified disarray, mouth webbed with strands of thick saliva, eyes a radioactive shade of yellow.” There is something primal and animalistic in his description, especially his trademark Sith eyes. They personify how the dark side corrupts the very being of a Sith.
The Jedi Path tells young Jedi padawans to always “remember that being a Jedi also means looking the part. Your attire is an outward sign of your commitment, and a constant reminder of your lifelong calling.” Their attire is a reminder to them, but it is also a symbol to everyone else in the Galaxy that they are Jedi -- the selfless servants who are willing to help.
As a direct contrast, Sith regularly choose clothing that is dark, menacing or concealing. The Emperor always wears a black Sith robe with his face partially obstructed by a dark hood, and Darth Vader’s whole suit is made up of dark metal and black fabrics. However, the list does not stop there: among others, Darth Maul, the Grand Inquisitor and Kylo Ren all choose to wear dark and concealing clothing. Even ancient Legends Sith like Darth Nihilus and Darth Traya are depicted as wearing black Sith robes.
In addition to dark clothes, many Sith choose to wear masks to present themselves as a symbol of darkness. The most notable Sith to wear a mask was the infamous Revan. After defeating the Mandalorians at Malachor V, he disappeared and fell under the influence of the Sith Emperor Darth Vitiate. He waged war against the Jedi before he was eventually redeemed. However, the Jedi took his mask away and wiped his memory because his mask was a reminder and a tangible representation of his former, evil self.
Another Legends Sith that wore a mask was Darth Nihilus, the Lord of Hunger. In order to retain a physical form, he infused his spirit into his mask. It defined him, and inspired terror in others as he literally fed on the Force. He was eventually killed by the Jedi Exile, but his mask became a relic of the dark side and gave power to its future wearers.
Many of the Sith have a mechanical component as well. Using the dark side of the Force often comes with a physical cost. It tests any Sith who tries to conquer its powers, and many pay the price. Darth Vader is the best example because his suit is what defines him and inspires fear. After his duel with Obi-Wan on Mustafar, Vader was left scarred, burned and missing multiple limbs, and he required extensive prosthetics and technology just to keep him alive.
Vader is not the only Sith to rely on mechanical advances to sustain life. Sith like Darth Malgus, Darth Plagueis and Darth Malek were all forced to wear respirators after their trials with the dark side had scarred them and taken away their ability to breathe. Even General Grievous relies on machines to keep him alive, and Darth Maul's legs were mechanical after his fight with Obi-Wan Kenobi. This shows that the Dark Side is unnatural, and those who use it often require mechanical assistance to just remain alive.