Let's make one thing clear: Palpatine isn't a sympathetic character. When Obi-Wan said, "Chancellor Palpatine is evil!" he was right. But what the hell did Palpatine think he was doing? As history and current events teach us, evil people don't usually think they are evil. Yes, Palpatine embraces the Dark Side, but he also says "good is a point of view." Assuming he thought he was doing good from a certain point of view, what was Palpatine's motivation?
Throughout most in-the-weeds discussions of Star Wars, we tend to focus on results and methods — Yoda and Obi-Wan's plan to hide the Skywalker twins, Luke's weaponizing of the oral tradition, Han getting frozen in carbonite by a familiar face — but sometimes we leave out the why. Who and What are fairly obvious when it comes to the bad guys of Star Wars, but the Why is often lost.
So, here's a thought: Why did Palpatine do everything he did? Possible answer: He was trying to balance the Force. Here's the evidence.
The notion of "bringing balance to the Force," for better or worse, pervades a huge part of how we try to make sense of the episodic Star Wars films. If we pretend that we didn't know these movies were written and directed by different people, and there aren't multiple retcons and narrative contradictions strewn throughout, then at least we can (maybe) agree that on some level, the Emperor is the only force-wielding character with consistent patterns (he wants to control the galaxy and kill the Jedi). And those patterns reveal someone who seemed to go out of his way to recruit people who were not pure evil. Why would he do that?
Simple. Like the Jedi, the Sith also want to bring balance to the Force, but they think the best way of doing that is by finding powerful, conflicted people and bringing those people over to the Dark Side.
Palpatine's preference for Light side apprentices starts with Anakin
Palpatine's least favorite type of Sith apprentice is someone really and truly evil. Think about it. Darth Maul and Count Dooku are way better at being unrepentant Sith Lords than Darth Vader or Kylo Ren. And yet, both of these guys are the types of apprentices Palpatine discards. He's never serious about Count Dooku remaining his one, true Sith apprentice. In The Phantom Menace, it feels like Maul is merely the muscle of the Sith, but not really the spirit of what Sidious is going for.
In that same film, Palpatine takes an early interest in Anakin Skywalker. This can be retroactively explained by that pesky in-canon Marvel comic book Darth Vader #25, in which a flashback scene seems to heavily imply that Palpatine influenced the midi-chlorians in Shmi's stomach create baby Anakin. This concept was walked back in 2019 by Lucasfilm representative Matt Martin, who claimed, "If the intention had been to create a direct connection between Palps and Anakin's birth, I would have had it removed." Still, even if Palpatine didn't create baby Anakin with the Force, it does seem likely he was aware of Anakin and, like the Jedi, sensed Anakin's potential to "bring balance to the Force."
From the (debatable) point in time at which he is aware of Anakin, Palpatine considers Anakin to be the best possible Sith apprentice. That's arguably not because Anakin has great potential for raw evil — that talent seems to be something that is pretty easy to find. Instead, Palps wants Anakin because of his potential to be both good and bad. Why would someone only interested in pure evil want to recruit someone like that? Simple answer: Palpatine knew the Force needed balance, too.
Palpatine wanted Luke Skywalker and Ben Solo for the same reasons
After Anakin is no longer useful to him (or dies) Palpatine, again, is on the lookout for a conflicted "good" person who could break bad. In Return of the Jedi, it's fairly clear that Palpatine wants Luke Skywalker, not because Luke is some kind of cold-hearted, murderous, Force-wielding badass, but because Luke's power comes from a blend of positive and negative impulses.
It feels pretty safe to say that most Skywalkers who become Jedi (Anakin, Luke, and Ben Solo) all have anger issues slightly rowdier than most people. It's also safe to say that all three of them have tremendous heroic qualities and a capacity for good. Two of them became warlords who killed other Jedi. One didn't. And if we buy that Palpatine was really Snoke all along, then that means he recruited Ben Solo pretty much exactly the same way he recruited Anakin.
This means that prior to the ending of The Rise of Skywalker, Palpatine has a pretty good track record with recruiting confused and duplicitous people to his cause. He got 2 out of 3! Sure, both Anakin and Ben Solo eventually turn on him, but during the time those guys are working for Palpatine, you could imagine that from the POV of Palps, that's what balancing the Force looks like: "See, I've got these do-gooders with hot tempers working for the Sith! I've balanced the Force!"
Rey was Palpatine's coup de grâce
At this point, we can see why Rey is the best possible apprentice Palpatine could ever hope for, and it's not just because she's his granddaughter. Like those Skywalker boys, Rey has a blend of endless empathy and straight-up rage. As we know by now, this is Palpatine's favorite combo. But because Rey is a Palpatine, her dip into the Light side is against her nature.
With the Skywalkers, turning to the Dark Side usually proves to not be their final decision. The point is, Rey is a lot like the Skywalkers in that she has potential for great evil or great good. But because she's related to Palpatine — the evilest person ever — the chances of her staying evil forever are higher.
This may not scan as true because of what Rey does in The Rise of Skywalker, but if we want Palpatine's plans to make any sense, then from where he sits (or hangs, I guess) the idea Rey could turn to the Dark Side and stay that way has to be more than a good bet. Luke Skywalker had nightmares he cut off Darth Vader's head. Rey had nightmares that she had monster teeth and sat on the throne of the Sith. Right there, you could argue Rey is at least thinking about being evil in more detail than Luke, Ben, or Anakin. Like Palpatine, Rey is kind of a loner just trying to make the galaxy a better place by running it. Rey wants justice for scavengers. Palps wanted justice for people on Naboo (maybe?).
Why did Palpatine's plan fail?
Assuming that Rey was actually the perfect apprentice for Palpatine, then why didn't it work out? There are a few practical reasons. For one thing, the Jedi don't fear death the way Palps does, which works to their advantage when they channel Force-ghost energy into Rey at the end of The Rise of Skywalker. For another, Palpatine always underestimates the pervasiveness of good impulses, because good impulses have a tendency to be more active and less lazy than evil. (Seriously, whenever Palps gets what he wants, what's the first thing he does? Looks for somewhere to sit down and chill out. And let's not even get started on the bathrobes.)
But, if we leave complacency and over-confidence out of it for a second, what else? If Palpatine kept getting apprentices with Light side tendencies in an attempt to balance the Force, why didn't it work?
Well, unlike Palps, the Jedi weren't trying to balance the Force through outright manipulation. When he encounters the Dyad of Rey and Ben Solo, he should have realized that he actually found what he wanted all along: the true balance between Light and Dark. But by that point, he's cloned himself so many times that he's lost sight of everything and fails to notice that the one factor that was keeping the Force out of balance was him trying to balance it. Palpatine was right to pick conflicted people with blended tendencies. He was right to think those kinds of people could balance the Force.
The thing he didn't think about was that he just might not be around to see it happen.