The end of George Lucas' prequel trilogy instead proves that Palpatine's face changes as part of his plan to manipulate Anakin, with Palpatine using the Force to hide his true visage up until that moment.
Trying to turn Luke to the Dark Side, he purposefully left himself vulnerable, making the Jedi feel like he was striking a defenseless old man. So, while it was tough to look at, Palpatine saw advantages in keeping his appearance the way it was.
Palpatine is certainly no stranger to hiding his appearance, as we've seen him do so in Star Wars Rebels. There's a popular theory, especially in the Legends continuity, that Palpatine used Sith alchemy to conceal his true identity behind a sort of mask, only for this mask to be torn away in Episode III.
Despite suffering a horrendous defeat and being very visibly disfigured, Palpatine was, first and foremost, a tactician. Instead of healing his face through the use of advanced technology or Sith alchemy, Palpatine left it in its mangled state and used it as a part of his ploy to publicly discredit the Jedi.
palpatine was a master of deception. and for years. he employed the use of a sith cloaking technique. known as a force mask.
Really, the yellow eyes of the Sith show the physical toll the dark side issues on its 'host' if you can call it so. It represents the dark power within them, the unchecked and even unbalanced rage swirling through their body. Kylo Ren was never fully immersed in the dark side, as was evident throughout the movie.
Of course, Sidious didn't tell Dooku the full plan, so that didn't work out too well for him. Regardless, Dooku's eyes weren't yellow because he wasn't motivated by anger. He was merely a practical dark side user, so he does not have the usual Sith eye color.
After his ascent to the position of Emperor, Palpatine dropped the charade and everyone saw him for what he was. His psychopathy was also prevalent even prior to becoming a Sith lord, evidenced by his delinquent behavior at an adolescent age, and later his committing manslaughter on two pedestrians via a speeder.
Numerous theories have attempted to explain this, among them being the idea the force lightning just exposed what years of succumbing to the dark side had done to Palpatine. Another theory is he purposely allowed the lightning to scar him so that he could appear more sympathetic to Anakin (Hayden Christensen).
Some have even speculated Sith can grow addicted to Force lightning. This naturally explains why Jedi do not generate Force lightning. Jedi seek to master their emotions rather than be mastered by them; Force lightning is therefore forbidden, as it is best understood as a spontaneous outpouring of hatred and rage.
1 Darth Sidious
As well as being a gifted bladesman Sidious' connection to the Dark Side is extremely powerful, granting Sidious the ability to conduct force lightning.
Qui-Gon Jinn was more attuned to the Force than any other prequel Jedi, and that's one reason why Emperor Palpatine needed him dead.
According to Star Wars Legends content, the first Sith was a former Jedi Master, Ajunta Pall, who became a so-called Dark Jedi, or “Fallen” Jedi.
While Mace truly won the fight, Palpatine was the more powerful combatant.
As for why he is evil, unfortunately he was just born evil. He gravitated towards the dark side and it enhanced all his worst traits. As others said, the Legends novel Darth Plagueis has information on it. As for why he is irredeemable, we actually know this.
As the Death Star had been destroyed, and the Battle of Yavin had finally taken place, the years then moved into ABY (After the Battle of Yavin). It's said that Star Wars: Return of the Jedi takes place during 4 ABY, meaning Palpatine was 88 when he met his original end.
Darth Vader can not use force lighting, because of his severed arms. Robotic hands can't summon the force lighting.
Vader relies on the complex life support system of his suit to survive and it happens to be very vulnerable to electrical discharges. So, if Vader ever had unleashed Force lightning, there's a good chance it would've caused his suit to short circuit, thus, killing him.
And therein lies the answer. Sith specifically channel Force lightning using their hands. Seemingly, because Vader's hands are no longer there, he can't channel the dark side of the Force through them, meaning that he can't use Force lightning.
In fact, in this same scene, Sidious says he is keeping Maul alive because he has "other uses for you!" If Palpatine thought Maul knew he about his double identity, there's no way he would have let him stay alive.
He once stood in the middle of the Jedi Council, and not a single Jedi Master (including Yoda) could sense the dark side in him. Moreover, he also used a technique known as “Sever Force,” to dampen the Force abilities of every single Jedi in the galaxy, with little to no effort as well.
Anakin Skywalker, one of the main characters in the "Star Wars" films, meets the criteria for borderline personality disorder (BPD).
Count Dooku became a Sith because the Jedi served a corrupt Republic, but he had another more personal reason to betray the order. The following contains spoilers from Yoda #4, on sale now from Marvel Comics. Darth Sidious was Star Wars' master schemer.
Once a Jedi -- trained by Yoda -- he became disillusioned with the Jedi Order and thirsted for greater power. Dooku voluntarily left the light side behind and became Darth Sidious' dark side disciple, taking the secret name Darth Tyranus and leading the Separatist army.
While Dooku was certainly evil -- and even had a Darth sobriquet, Tyranus, that he rarely used -- he was never quite a Sith, at least not in the true sense. Unlike those who espoused the Sith doctrine, Dooku wasn't motivated by hatred or fear, but by the same purpose that drove him as a Jedi.