One reason could be to add more conflict within the Weasley family. Percy was already somewhat of an outcast in the family due to his loyalty to the Ministry. Turning him into a Death Eater would intensify that divide.
In fanfiction, Percy Weasley could be a Death Eater to explore the concept of redemption. If he starts as a Death Eater, the story could then focus on his journey back to the light side. This could be a very character - building arc. Also, some authors might make him a Death Eater just for the shock value. It's a way to grab the reader's attention and make them question everything they thought they knew about the character and the story world. It also allows for different interpretations of the wizarding society's values and how easily they can be bent or broken.
It might be to show the corrupting influence of power. Percy was ambitious in the original story. In fanfiction, this ambition could be exploited by the Death Eaters, leading him to join their ranks. This would also serve as a warning about the allure of the wrong kind of power. Another reason could be to create new and unexpected alliances. For example, if Percy becomes a Death Eater, how would his relationship with characters like Hermione or Harry change? It opens up a whole new world of plot possibilities.
In some fanfictions, Percy Weasley may be depicted as a Death Eater, but this is a departure from the original canon. Fanfiction allows for creative reinterpretations, and some authors might explore this dark turn for his character to add drama and new plotlines.
Some fanfiction writers might make Sirius Black a Death Eater to explore the idea of redemption in a different way. If he starts as a Death Eater, then his journey to the side of good could be a more complex and interesting story. Also, it could be a way to add more mystery to the 'Harry Potter' universe by changing the nature of a familiar character.
One reason could be his regret over his past actions. For example, when he distanced himself from his family to pursue his career at the Ministry. He might realize later how wrong he was and cry out of remorse.
Perhaps it's a symbol of his new - found independence. After leaving his family and then potentially coming back, he might be exploring new things. Smoking could be part of that exploration, something that he wouldn't have considered before. Also, it could be used to add an air of mystery or edginess to his character. Fanfiction writers often use such elements to make a familiar character seem more complex and interesting.
Another possibility is that he is in love with her. In fanfiction, love can be a powerful motivator. He might be attracted to her intelligence, her strong will, and her beauty. This love could override his loyalty to the Dark Lord and make him want to protect her at all costs. He could see her as a symbol of hope and a better future for the wizarding world.
It could also be that Hermione is on a mission to gather information. By helping the Death Eater, she hopes to gain insider knowledge about the Death Eater's group or their plans. This would be a strategic move on her part, using the situation to her advantage in the fight against the dark forces.
Family drama. The Weasley family has its fair share of chaos. His relationship with his family members, especially after some of the disagreements he had, could be a source of emotional stress that manifests as migraines. For example, his estrangement from his family during a certain period in the books could have taken a toll on his mental well - being.
It could be because Percy is different. If he comes from a different background or has a unique personality trait, the bullies might target him. In the fanfiction, if Percy is from a different race or has an unusual way of speaking, the bullies might pick on him for not being like them.
Love could be the main reason. Since he loves Annabeth deeply, he doesn't want to lose her, so he becomes possessive.
One reason could be for a fresh take on the story. Writers might want to break away from the typical demigod narrative and explore a darker, more supernatural side of the Percy Jackson universe. Another reason could be to create a more complex and morally ambiguous character. By making him a demon, they can introduce new conflicts and relationships.