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.
I'm not sure if there is a specific 'ash no pikachu fanfic' out there. There are countless fanfics in the Pokémon fandom, but this particular one might be hard to find. You could try searching on dedicated fanfic websites like FanFiction.net or Archive of Our Own and use the relevant keywords to see if it exists.
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.
It's doubtful that it's a popular fanfic. Popular fanfics usually tend to build on the positive aspects of the source material, like the friendship or adventure between characters. A concept where Ash kills Misty is so out of the ordinary and negative that it probably doesn't appeal to a wide audience. However, there might be a small niche of readers who are interested in very dark and alternative fanfics, but overall it's not likely to be popular.
In a fanfic, Ash might tell Gary a spooky story about an Electrivire that went berserk. With Pikachu by his side for added effect, Ash could describe in vivid detail how the Electrivire was uncontrollable and its power was overwhelming. Gary, being a competitive trainer, would be both scared and intrigued, wondering if he could ever handle such a Pokémon. This could play on Gary's insecurities as a trainer and create a great story of fear and rivalry.
I'm not sure if there is an exact 'ash x pikachu lemon fanfic' available. Fanfics can be quite diverse, but this combination seems rather strange as Pikachu is a Pokémon. Most Pokémon fanfics focus on more common relationships within the Pokémon universe, like trainer - Pokémon partnerships in a non - strange way.
One possible plot could be that Ash starts his journey with a different starter Pokémon, like Bulbasaur. Without Pikachu by his side, he might face different challenges and form different relationships. He could struggle more in the early battles without Pikachu's electric power, but then develop unique strategies with his new Pokémon team.
I'm not sure specifically what this 'ash kills misty fanfic' is about as there could be many different interpretations. It might be a very specific and perhaps dark or alternative take on the relationship between Ash and Misty in a fan - created story.
In this fanfic, it could be that Ash is turned into Pikachu as a result of an experiment gone wrong. As Pikachu, he discovers new abilities and senses that he didn't know about before. He might form closer bonds with other Pikachu in the wild and learn their ways of living. He could also have to face challenges like being hunted by other Pokémon that see him as an easy target, and he has to use his new Pikachu - like skills to survive and try to find a way to turn back into his human form.
The 'ash meets pikachu fanfic' often creates new scenarios for their adventures. For example, they might end up in a different dimension where the rules of Pokémon battles are completely different. This allows for a fresh take on their relationship and how they adapt to new situations.