One possible motive could be extreme revenge. After years of being mistreated, locked in a cupboard, and generally being made to feel unwanted, Harry might snap in fanfiction. Another could be a sense of self - preservation if the Dursleys were somehow involved in a plot against Harry's life in the fan - created plot.
In some fanfiction, the Dursleys could be working with dark wizards against Harry. This would give Harry a reason to kill them in self - defense or to stop a greater evil. Another motive could be that the Dursleys finally pushed Harry too far with their constant degradation and neglect. Harry, having reached his limit, might take drastic action in the fan - created scenario. Also, if the fanfiction explores a darker version of Harry's character, he might kill them just because he has the power to and wants to assert his dominance over those who once oppressed him.
Well, perhaps in the fanfiction, the Dursleys might have discovered Harry's magic and tried to harm him in a more serious way, like trying to permanently suppress his magic. This could lead Harry to kill them out of a need to protect his own identity and future as a wizard. Or it could just be the result of years of built - up hatred due to their emotional and physical abuse.
One possible motive could be revenge. The Dursleys treated Harry awfully for years, so in fanfiction, he might reach a breaking point. Another could be self - defense if the Dursleys in the fanfiction were to take their abuse to an extreme level.
One possible motive could be misinformation. If Harry is fed false information about Sirius being a traitor or a danger, he might act on it. For instance, if a well - disguised Death Eater manages to convince Harry that Sirius has joined Voldemort, Harry, in his haste to protect others, could attack Sirius.
One possible motive could be a misunderstanding. Maybe Snape is framed for something really bad, like a dark magic plot against Hogwarts. Harry, in his haste to protect the school and his friends, believes Snape is guilty and takes drastic action. Another reason could be if Snape accidentally endangers someone Harry loves dearly, like Ginny or Ron, and Harry's emotions get the better of him in the heat of the moment.
Maybe Fudge was starting to turn against the wizarding community in more extreme ways. For example, if he was planning to persecute Muggle - borns more harshly or side with the Death Eaters secretly. Harry, being a defender of justice and equality in the wizarding world, would not tolerate such actions and might have been pushed to kill Fudge to stop his evil plans.
One possible motive could be extreme revenge. If the Muggle relative, like the Dursleys, had been particularly cruel to Harry over the years, some fanfiction writers might explore the idea of him snapping and seeking ultimate revenge. Another could be a dark magic influence. Maybe in the fanfic, Harry is exposed to some dark magic that corrupts his mind and makes him view his Muggle relative as an obstacle to be removed.
He could be doing it out of a sense of justice. Despite his family's views, Draco might see that the way the Dursleys treat Harry is wrong and decides to step in.
Another motive might be related to a dark object influencing her. If Hermione came across a cursed object, like a dark locket that amplified negative emotions such as envy or fear, it could drive her to kill Harry. Maybe she started to envy Harry's fame and the special attention he always got, and the locket pushed her over the edge.
One possible motive could be that Snape is still deeply influenced by his loyalty to Voldemort in this fanfiction. Maybe Voldemort has some hold over him that forces him to do the unthinkable. Another reason could be a misunderstanding. Snape might believe that Harry has turned to the dark side or is about to, and he decides to take preemptive action to stop what he thinks will be a great evil.
One possible storyline could be that the Dursleys' pet, perhaps their cat, discovers Harry's wizarding abilities before the Dursleys do. It might then have some comical or tense interactions with Harry, like trying to stop him from using magic in the house for fear of getting in trouble itself.
One possible plot could be that the Dursleys receive a threat from the wizarding world that they can only be protected from if Harry stays with them and helps. Harry, being a good - hearted person, decides to do so and in the process, he starts to understand them better and grows to love them.