Well, the Dursleys could manipulate Harry's view of the wizarding world. They could tell him lies about how all wizards are evil and that he should stay away from them. They could also destroy any letters or signs from the wizarding world that come to Harry. By doing this, they cut him off from the only positive community he could belong to. As a result, Harry, feeling alone and betrayed, might start to develop a hatred towards both the wizarding world and the Dursleys, and this could lead him down a dark path.
The Dursleys could start by being even more cruel and abusive. They could constantly belittle Harry, tell him he's worthless and that he doesn't deserve any kindness. This could chip away at his self - esteem and make him more vulnerable to dark thoughts.
One key theme could be the corruption of innocence. The Dursleys, who are typically neglectful and mean in the original, might take their actions to an extreme in a dark fanfiction, completely warping Harry's view of the world. Another theme could be power dynamics. They might use their position as Harry's guardians to gain some sort of dark power or influence. Also, isolation could be a theme, as they further isolate Harry from any positive influences, leading him down a dark path.
They could also physically and emotionally isolate him even more than they do in the original story. By cutting him off from any positive influence, like his friends at Hogwarts, they make him more vulnerable. Then, they could introduce him to some seemingly 'cool' or 'powerful' dark magic artifacts or spells, which might initially attract Harry's curiosity and eventually lead him down a darker path. For instance, they might find a way to get their hands on a dark magic book and leave it for Harry to discover.
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.
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.
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.
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.
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.
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.