The 'Stiles is a badass' fanfiction might be interesting because it gives a new perspective on the character. Often in the original stories, Stiles might be seen as more of a sidekick or the brains behind the operation. But in the fanfiction, he could be transformed into a fearless fighter. His quick thinking, which was already a trait in the original, might be enhanced and used in more action - packed and badass ways.
Well, it could be many things. Stiles is a character with a lot of potential for badassery. Maybe in the fanfiction, he has extraordinary skills or a really tough attitude that wasn't fully explored in the original work. It could also be the unique situations the author puts him in, like facing off against powerful villains all by himself and coming out victorious.
One reason it's interesting is that the fanfiction might develop Stiles' backstory in a way that makes him a badass. For example, it could reveal that he has some secret training or a hidden heritage that gives him an edge. Also, the relationships he forms in the fanfiction could contribute to his badass image. If he teams up with other characters in a new and unexpected way, it can make for a really engaging read.