The My Hero Academia: Vigilantes anime recently depicted how Stendhal, the murderer of villains, turned into the killer of heroes in the original series, Stain, which was mostly disappointing to a lot of people. That is because the character's backstory was something that most of the fandom wanted to see and what was shown in this spin-off didn't live up to the expectations of many.
The main issue surrounding Stain's origin in My Hero Academia: Vigilantes is that he was driven to become a vigilante because of his admiration for All Might, but this spin-off doesn't give that or his disappointment with the hero society any focus. If anything, the original series gives more information and context surrounding what drove him to become who he is than the series that shows his actual descent into Stain, with the latter feeling more like fanservice.
Disclaimer: This article contains spoilers from the manga/anime and reflects the opinions of the writer.
Explaining why Stain's origin in My Hero Academia: Vigilantes doesn't do justice to the character

The most recent episodes of the My Hero Academia: Vigilantes anime feature Stendhal, a vigilante who takes the lives of criminals, which ends up causing him to fight with Koichi and Knuckleduster. However, as the latter manages to overpower him and they have a discussion about determination, this becomes the main driving force for Stendhal to turn into the hero killer of the original series, Stain.
That this twist was a disappointment to the fandom would be an understatement, especially considering how this spin-off's title was supposed to focus on characters such as Stendhal/Stain. This origin doesn't provide more context for his motivation and how he started to fight villains, to the point that this could be removed and wouldn't have changed anything regarding his character's past.
A major problem with this is that Stendhal already appears as someone taking the lives of villains and speaking in a rather sophisticated and complex manner, suggesting that he tries to portray a certain image. It would have been interesting to see his backstory, but alas, the spin-off only treats his appearance as fanservice since he is not going to appear anymore in this series.
The many elements of Stain's character

A lot of fans were expecting My Hero Academia: Vigilantes to give a lot more information about characters like Stain because he is someone who operates outside of the law with a very peculiar view of hero society. Therefore, that is why his origin should have provided more details about who he was and what drove him beyond a somewhat abstract adoration of All Might that is never fully explored.
The last part might be the most telling regarding Stendhal in this series because nothing is truly explored regarding his character, which is why he ends up feeling disappointing and underwhelming all things considered. This is especially telling because his leap of logic regarding how he went from murdering villains to killing heroes also needed more context, thus making this entire situation all the more frustrating.
Final thoughts
My Hero Academia: Vigilantes missed an opportunity to explore Stain’s character more deeply, especially his respect for All Might and how it ties into the spin-off’s themes. Overall, his backstory feels disappointing, which has been a common criticism of the series
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