Maybe the creator wants to explore a different side of Pikachu's character. In the official story, Pikachu is loyal and friendly, but in this fanfiction, it could be a way to show a corrupted or influenced Pikachu. Also, it could be a way to deconstruct the typical hero - sidekick dynamic between Ash and Pikachu and make readers think about their relationship in a new way.
Another reason might be related to Pikachu's own growth. As Pikachu becomes more powerful and self - aware, it might feel that it needs to go on its own journey without Ash. It could be seeking new challenges and experiences that it believes Ash can't provide, like exploring regions that Ash has no intention of going to or training with Pokémon of a different caliber.
One possible reason could be that Ash senses a dangerous situation where Pikachu's full power might attract unwanted attention or put them in more danger. For example, if there are powerful and unseen enemies nearby, Ash might hold Pikachu back to keep a low profile.
I haven't come across a 'pikachu kills ash fanfiction' myself. But in the world of fanfiction, people sometimes create very extreme and unexpected scenarios. If such a fanfiction exists, it may be written by someone who wants to explore the darker side of the Pokémon universe or perhaps is trying to make a statement about the relationship between Pikachu and Ash. It might shock and intrigue readers who are used to the positive and heroic portrayals of the characters in the official Pokémon media.
Some authors might create it as an exercise in 'what - if' scenarios. They want to break the established narrative and see how different characters would react if Harry was killed so early. Also, it could be a way to explore the Dursleys' characters more deeply, showing just how far their hatred for Harry could go.
One possible reason could be to create shock value. Writers might want to break the expectations of readers who are familiar with the loving relationship between Edward and Renesmee in the original 'Twilight' story. By creating such a dark and unexpected scenario, they hope to grab the attention of readers who are looking for something different from the usual fanfiction.
Perhaps Pikachu is lured away by something more appealing. There could be a rare and powerful Pokémon or a special kind of food that distracts Pikachu from protecting Ash. Also, Pikachu might have received some false information. For example, it might have been tricked into thinking that Ash has a better chance of escaping the Spearow without it. In the heat of the moment, Pikachu makes a hasty decision based on this wrong information.
In some fanfictions, it could be related to a power struggle within the wizarding world. If Hermione was seen as a threat to Ginny's family or her own standing in the magical community, Ginny could be pushed to the edge. Also, if there was a rift in their friendship over ideological differences, like Hermione's more logical and rule - based approach clashing with Ginny's more impulsive and instinctive nature in a very extreme way, it might lead to such a tragic outcome.
One possible reason could be a misdirected spell during a battle. In the heat of a fight, Harry's spell could go awry and accidentally kill Malfoy. Another reason might be if Malfoy was under the control of a dark wizard and was about to do something truly terrible, and Harry had to stop him at all costs, including taking his life.
Well, it could be a culmination of events. Uther's growing distrust of those around him, combined with Merlin's secretive nature about his magic. Merlin might be caught in the middle of a power struggle or a plot against Uther, and Uther, in his quest to protect his throne and Camelot, decides to eliminate Merlin without fully understanding the consequences. This would also create a great deal of drama and conflict for the other characters in the story, which is something fanfiction writers often strive for.
A plot twist could be that Ash's newfound 'Pikachu - fandom' causes a rift between him and his other Pokémon. They feel neglected because he is so focused on Pikachu. But then, through a series of events, Ash realizes that he needs to balance his love for Pikachu with his care for his other Pokémon friends. This realization could lead to a more harmonious relationship among all of them in the end.