Well, one possible reason could be their ignorance of the magical world. Muggles, not understanding magic, might accidentally trigger events that lead to destruction. For example, they could unknowingly interfere with magical spells or artifacts that are crucial for maintaining the balance of the world.
It might show a relationship full of misunderstandings. Muggles are unaware of the magical world, and the magical world has to keep its secrecy. So there's a lack of communication, which can lead to problems later.
It could be that in the fanfic, Harry Potter discovers some deep - seated secrets about the wizarding world's power structure. Feeling betrayed, he decides to take extreme actions to expose and destroy it. For example, if he finds out that the Ministry of Magic has been using unethical means to control the wizarding population, he might use his influence and magic to bring it all crashing down. He could start by revealing the truth to the public, which could lead to chaos and the destruction of the established order in the wizarding world.
One possible reason could be extreme anger. If something really bad happened to his loved ones, like if Voldemort came back in a different way and killed all his friends and family this time, Harry might be driven to the edge and decide to destroy the wizarding world that couldn't protect them.
Their underdog status. Muggles in the Harry Potter world are seen as inferior in terms of power compared to wizards. So when a muggle is 'badass', it's like a David vs Goliath situation. They have to rely on their non - magical skills and wits to overcome magical challenges, which is really exciting to read.
In such fanfiction, Muggles can be shown in different lights. Some portray them as simple and uncomplicated, living their ordinary lives without any knowledge of magic. However, there are also fanfictions where Muggles are more curious or even a bit envious of the magical abilities of Wizards. They might be depicted as trying to find out about the wizarding world despite the secrecy.
In the 'Harry Potter' fandom, this would be a very controversial concept. Dumbledore is often seen as a mentor and a powerful, positive figure. Having Harry Potter destroy him goes against the general perception of their relationship in the original series. However, in the realm of fanfic, some might explore such a dark and alternative take as a way to examine different character arcs and what - if scenarios, but it would likely be met with a lot of resistance from purist fans.
One 'badass muggle' could be a character who manages to outwit dark wizards using just their street smarts. For example, in some fanfics, there are muggles who are really good at martial arts and they use their physical skills to hold their own against wizards who underestimate them.
One possible reason could be a misinterpretation of his character. In the original series, Harry Potter doesn't hate Muggles at all. But in this fanfic, perhaps the author wants to explore a 'what if' scenario where something drastic has happened to make him have such feelings. Maybe he's been betrayed by a Muggle - in the fanfic world, this could be a made - up situation that sours his view on all Muggles.
No, they are not very popular. The original 'Harry Potter' series is loved for its themes of magic, good vs. evil, and coming - of - age. Fanfictions that involve 'Harry Potter and Muggles' in a lemon context often go against the general tone of the series. Also, the official 'Harry Potter' brand promotes a family - friendly environment, so such fanfictions with adult - like content don't really fit into the mainstream popularity of the 'Harry Potter' fandom. They are more of a niche and often not well - received by the general fan community.