They would likely be disgusted. The Dursleys are so anti - magic that any form of fan - created stories about the magical world would be abhorrent to them.
The Dursleys' reaction would be complex. Vernon would be enraged and might even try to burn any fanfiction he found. Petunia would be conflicted. Part of her would be disgusted, but a small part might be a bit curious as she has some connection to the wizarding world through Lily. Dudley might be more interested than his parents, especially if the fanfiction had some action or adventure that appealed to his more adventurous side.
They'd probably be horrified. The Dursleys are very much against the idea of magic, and fanfiction might introduce even wilder ideas about the wizarding world.
Hermione would be heartbroken. She would probably try to find out the exact cause of Harry's death, suspecting the Dursleys' negligence or worse. Ron would be furious with the Dursleys and would want justice. Dumbledore would be deeply saddened as he had placed so much hope in Harry. The Weasley family would also be in mourning, having considered Harry as one of their own. And Voldemort would be triumphant, thinking that his greatest obstacle was removed.
The Ministry of Magic would be in a difficult position. On one hand, they would need to keep the wizarding world calm. On the other hand, they would have to deal with the fact that a muggle family killed a famous wizard. There would be debates about how to handle the Dursleys. Some might want to obliviate their memories and send them to Azkaban, while others might think that a more public trial is necessary to show that the wizarding world does not tolerate such actions against one of their own.
The wizarding world would likely be in shock. Harry was seen as a hero, and this act would completely change their perception of him. Some might think he was finally getting revenge for all the abuse he endured, but many would condemn his actions as murder is still a serious crime in the wizarding world.
One way the Dursleys can be good in fanfiction is if they are forced to face the consequences of their actions early on. For instance, they might receive a warning from the wizarding world that if they don't treat Harry well, there will be serious repercussions. So they start to be nicer to him. They could provide him with a proper room, better food, and allow him to have friends over.
In some Harry Potter fanfictions, the Dursleys are depicted as nice. This could be a really interesting take. It might show how they could have been different if they hadn't been so influenced by their dislike for magic. Maybe they could have supported Harry more, or at least been less cruel. For example, a fanfiction could explore a scenario where Vernon Dursleys actually encourages Harry's academic achievements at school, instead of always putting him down.
It might show the constant power struggle between them. Harry, as a wizard, has an inner strength that the Dursleys try to suppress with their mundane and cruel ways. So it's a battle of wills.
The Dursleys could also cripple Harry emotionally in fanfiction. Their constant belittling, isolating him from others, and treating him like a burden could break his spirit. This emotional crippling might make it difficult for Harry to trust people, form relationships, or even fully embrace his wizarding identity. He may become withdrawn and lack the confidence needed to face the challenges in the wizarding world.