Emperor Palpatine is undeniably the biggest villain of the Star Wars universe. Throughout all three cinematic trilogies, he's the reason that pretty much everything horrible happens. However, in Star Wars: Revenge of the Sith, Palpatine is almost stopped for good when Mace Windu turns his own Force Lightning down on him. If not for Anakin's interference, Palpatine might have been slain then and there before he could conquer the Republic, turning it into a Galactic Empire.
However, this all assumes that Mace Windu would have actually been a good leader. A recent fan theory posted by ImpulsiveBeetle on the r/FanTheories subreddit proposes that whoever won that fight would've become a galactic conqueror and that Mace Windu could've been as tyrannical as Palpatine.
The Theory
This theory suggests that Mace Windu might have been corrupted by power during the Clone Wars. The Prequels show how the Jedi exist as peacekeepers who maintain order. While historically there have been wars in the galaxy, the Jedi don't exist as generals -- at least, not regularly.
However, Mace Windu os a major wartime figure for the Jedi, and he orchestrates the Republic's actions during the war, not unlike Palpatine. While he fights to end the war, he states that, in order to secure a peaceful transition, the Jedi will need to control the Senate. This indicates that, much like Palpatine, he sees the functions of the Republic as an unnecessary measure and that things would run more smoothly if the Jedi ran things.
The parallels between Windu and Palpatine continue into the two's duel. Both Palpatine and Windu state that their adversaries are "too dangerous to be kept alive." Both of these statements show that the characters disregard the functions of democracy and law, especially when it comes to disposing of adversaries.
The theory culminates by stating that, following the Clone Wars, Windu intended on restructuring the Senate to function under Jedi control in order to maintain peace on Windu's terms. Palpatine, who retained absolute power over the Senate, served as the biggest obstacle to Windu's agenda, which was made all the worse when Windu realized Palpatine was a Sith Lord. Even during his fight, Windu multiple times mentions how Palpatine has total control of the Senate and the Courts. This knowledge leads Windu to conclude Palpatine is too dangerous to keep alive. Windu isn't alarmed by Palpatine since he's strong in the Force, only that he's in control of the political system.
Strong with the Dark Side
Another component of the theory is that Windu is the only Jedi other than Anakin who is strong in both the Light and Dark Sides of the Force. While the theory doesn't elaborate beyond describing how Windu's purple lightsaber represents the darkness and light within Windu, there are in-canon expansions on Windu's Dark Side experience.
Windu is a master of Form VII lightsaber combat. Every practitioner, other than apparently Windu, has fallen to the dark side due to how it channels aggression and anger when fighting. It seems odd that Windu is the exception to this fighting style's legacy.
In the canon novel Dark Disciple, Windu suggests assassinating Dooku, which Obi-Wan and Yoda oppose out of fear that such an act would draw them far closer to the Dark Side. Windu never argues that this wouldn't, but tries to push the practical reality of such an act and how it would further the Jedi's campaign against the Separatists. His actions result in Quinlon Vos, the Jedi sent to kill Dooku, falling to the dark side. This set a precedent for Windu's actions leading people down dark paths.
Dark Side users often have a lust for power, usually to satisfy some emptiness or rage within them. We see this in every Sith in the series. We see this rejection of power in every Light Side user, including the majority of the Jedi Order. It is possible that Windu was descending into the Dark Side over the course of the Clone Wars, only to be little better than a Sith by the time he fights Palpatine, where he visibly draws on his rage to beat the Sith Lord.
How Plausible is This Theory?
All in all, this theory is surprisingly robust. While Windu is presented as a Jedi Master, it's undeniable that he has contempt for the political system, believes that the Jedi should run the political show for the good of the people and undeniably has strong ties to the Dark Side.
However, the theory does have notable flaws within it. The first is that Windu and the Jedi are on the same page about Palpatine: Windu is far from the only person who thinks that he has too much power and needs to surrender. The Jedi, Windu included, tried to use diplomacy and espionage to uncover Palpatine's true agenda before taking any dangerous action. While it is possible that Windu's actions were motivated from some sense of power-lust, it's more likely that he was motivated by a desire for self-preservation. Windu first confronted Palpatin to arrest him, not to kill him.
On top of that, unlike Palpatine, Windu doesn't take any action earlier in the prequels that can secure the authority of the Jedi. In fact, one of Windu's biggest flaws throughout the series is how little he acts when he needs to. As Luke points out in The Last Jedi, the Jedi Council's failure to act led to Darth Sidious rising to power. If Windu were trying to conquer the Senate, he might have acted more directly rather than languish and discuss while the Sith rose to power.
After Windu watched multiple Jedi being felled by Palpatine's lightsaber, he undeniably saw him as a dangerous threat. Perhaps the dark side did influence Windu's thought process, but it is more likely that Windu was motivated more by wrath than power-lust. Form VII combat channels rage and anger as a weapon. The reason so many practitioners of his style fall to the Dark Side is because they let anger and hate command their actions. Arguably, this makes Windu even MORE dangerous than Palpatine, because Windu is willing to defy law and order out of irrational anger rather than calculated power-lust.
It is undeniable that Mace Windu wanted to take control of the Senate after the Clone Wars. However, it still seems unlikely that he would've wanted to retain control over the Senate after the transition from war to peace.
