So I know Cinder has a bit of a...reputation, both in community and out, as being one of the most despised characters in...seemingly all of fiction. I'm just going to say it right now, the hate is undeserved. While definitely unable to hold a candle to pretty much every other villain in RWBY, I honestly think Cinder is extremely underrated and overhated, and I...can't for the life of me figure out WHY.
Here's the thing, Cinder is MORE interesting in 4 onwards than 1-3. In 1-3, she's just the one in charge who acts all menacing and mysterious. In 4 onwards, she's motivated by revenge and spite, along with trying to gain Salem's favor in later Volumes, not to mention that the fact she has a Grimm arm instead of a prosthetic made by Watts always caught my attention for the potential arc implications. I always thought Cinder had the most potential out of any villain in this series, having solid rivalries with 2-3 different characters and an interesting question as to her fate thanks to the Grimm Arm. The one and only thing 1-3 has over 4 onwards is a more interesting fighting style, and that was back during Monty's time animating the fight scenes, so I legit think that's an unfair standard to hold the rest of the fights to (Though, funny enough, one I think a few fights after 3 actually meet).
All this to say, I think the idea that Cinder peaked in 1-3 and went downhill as a character afterwards is a bit backwards, and steeped in the more broad hatred of nearly anything after 1-3. I binged this entire show in like a week, there is no noticeable drop in quality between 3 and 4, in fact I think 4 was immediately living up to 3's general quality in that it introduced Watts, AKA my favorite character and the only villain I like more than Torchwick, in the first scene of the first episode. I thought Cinder's characterization made sense, given how Ruby blasted her arm off and burned half her face to a crisp, and as such humiliated her in front of the other three of Salem's Inner Circle. I think Cinder wanting revenge and to prove herself in addition to wanting the Maiden Powers made her a more interesting character overall.
So I suppose I should actually ask the question in the title, why IS Cinder so hated? I legitimately don't think it can be her personality, since as stated before, she has more personality in 4 onwards than 1-3. If you think she was boring in 4 onwards, then I wonder why that is compared to 1-3. If it's just intelligence, then what needs to be understood is that Cinder is no longer the string-pulling Main Villain. When it was revealed that Salem was her boss, and that Watts made the Black Queen Virus and is in turn basically Salem's Second-in-Command, I thought the idea of continuing to try and sell Cinder as the true mastermind behind everything was a bit laughable. RWBY in general went far beyond the scope of just fighting criminals, and I imagine that's what a lot of people don't like about it.
I would pose the theory that Cinder is so hated because she killed Pyrrha. That would make sense, Pyrrha was an extremely beloved and badass character who seemed like she had a lot of potential (Need I remind you, she fought Cinder, a Maiden, on pretty much equal footing, her title is NOT just a lie her Semblance told), she was a fan-favorite and had a really great arc with Jaune. This seems like the perfect reason...
...Except that Äs Nödt and Sukuna exist.
Now while I can't speak much on Sukuna since...I haven't yet watched Jujutsu Kaisen and don't know how Sukuna is viewed at the moment, I can speak on Äs, and let me tell you, if you think the backlash to PYRRHA's death was insane, then you haven't been catching up on Bleach for the past decade. Basically, Äs Nödt killed Byakuya. THAT Byakuya. Byakuya Kuchiki. While this is apparently just a rumor, if it's true, then it blows the hatred of Pyrrha's death OUT OF THE WATER. When Byakuya died, many of his fans THREATENED TO KILL THE MANGAKA, and did so so ferociously that he actually brought Byakuya back and left social media for a long time, even urging other writers to also not use social media. This was roughly ten years ago, presumably long before what Twitter/X has become today.
Äs Nödt, is one of the most popular Sternritter to this day.
That is why I can't really cite this as the definitive reason. Byakuya was around for years at that point, and had a much larger fanbase than Pyrrha, yet Äs Nödt nearly killing him didn't tarnish his reputation in the eyes of the general Bleach fandom, in fact, his crushing defeat of Byakuya is often praised to this day as not only his defining moment, but also the defining moment of nearly the entire First Invasion. And guess what? From an objective viewpoint, Cinder still has more personality than Äs Nödt. I am saying that, as someone who considers Äs Nödt one of my favorite characters in fiction.
Of course, there are a few other reasons. Her doing a couple fakeout deaths is often pointed at, but honestly that's nothing new for fiction or even RWBY specifically. Her backstory is also there. I don't see anything wrong with it aside from knowing what it could've been, flipping the Cinderella story on its head to show that Cinder was cruel and callous even back then, in spite of a loving family. I also don't think it can be her motivation, revenge is a pretty well-liked motivation, as is wanting to prove oneself, and even if we just talk about stealing the Maiden Powers for their own sake, Ozai wanted to take over the world for its own sake and I'm sure damn near no one will say that Cinder is better than Ozai.
Of course, it could be the combination of all these things, but I still don't think that's enough for the reputation she has. Cinder is more hated than PePe Waccabrada, and it is literally a meme in the Bleach community that everyone despises PePe's very existence as a character and sees him as nothing more than a walking joke. I cannot think of a single character more hated than Cinder, and considering that there are likely literal THOUSANDS of characters objectively worse than her in every way, I think that's extremely sad.
So, no, I don't know why she's so hated. Part of me thinks the most likely explanation is that, like with PePe, it just became a meme to hate her, that it's just the bandwagon everyone jumps on if they have the slightest gripe with her, and I'm someone who considers Cinder average in terms of quality, not a character I particularly like or care that much for, again she's one of my least favorite villains in this show though not really through much fault of her own, but also not a character who deserves nearly this much hate.
Thank you for coming to my rant, whoever had enough time and patience to read this whole thing.