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.
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 He may become withdrawn and lack the confidence needed to face the challenges in the wizarding world.
No. In the original 'Harry Potter' series, the Dursleys are presented as Muggles who dislike anything related to magic. Fanfiction is a concept more associated with fans of the series, and the Dursleys would have no interest in it.
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.
A lot of fanfictions play around with the Dursleys' relationship with Harry. They could be depicted as having a sort of begrudging respect for Harry. For instance, when Harry starts to show real courage and power, the Dursleys might start to see him in a new light. This is different from the original story where they mostly just try to suppress Harry. Also, some fanfictions might give the Dursleys magical abilities of their own, which completely changes their dynamic with Harry and the wizarding world.
In some fanfictions, the Dursleys might be portrayed as having a change of heart. For example, they could start to secretly admire Harry's magical abilities instead of resenting them. This can add an interesting twist to the original story.
One factor that can make the Dursleys good in fanfiction is guilt. They might feel guilty for how they've treated Harry over the years. This guilt could drive them to change their ways and be more loving and accepting towards him.
Often, a good Dursley in fanfiction is the result of a different backstory. Maybe Petunia has a moment of reflection about her sister Lily and decides to treat Harry better. Vernon could also come around when he sees the power and influence that Harry might have one day and wants to be on his good side. So, in these fanfictions, they become more likeable and positive characters.
I haven't actually come across a 'Dursleys kills Harry Potter' fanfiction. It's a rather disturbing concept considering how the story of Harry Potter is centered around his survival against Voldemort. The Dursleys are just part of his Muggle upbringing with their own brand of meanness. If there were such a fanfiction, it would probably change the whole trajectory of the story. For example, the wizarding world would lose its Boy Who Lived, and it would be interesting to see how the characters like Dumbledore or Hermione would react to such a tragic event.