Well, a plot might involve Leaf betraying Ash because she becomes jealous of his growing bond with a new Pokémon trainer. She feels left out and in a moment of weakness, she gives away Ash's battle strategies to his opponents. Or perhaps, Leaf is deceived by a false prophecy that says Ash will bring doom to the Pokémon world, so she tries to stop him by betraying their friendship.
It could be a new and powerful trainer who has a vendetta against Ash. This trainer might see Ash as a threat to their own rise to the top in the Pokemon world. They could manipulate Leaf into betraying Ash to further their own goals.
Perhaps Leaf betrays Ash because she becomes jealous of his friendship with Pikachu. She might feel like Pikachu gets all of Ash's attention and decides to side with a group that wants to separate them. This could lead to Ash having to fight not only to save Pikachu but also to win Leaf back.
I'm not entirely sure specifically about this 'evil ash betrayed fanfiction' as there could be many different interpretations. It might involve the character Evil Ash in a situation where he betrays someone or something within the context of the fan - created story.
The betrayal in the fanfiction could have a big impact on the Pokémon world. Ash has many Pokémon friends and allies. If Leaf betrays him, it could lead to some Pokémon being captured or in danger. Also, the bond between humans and Pokémon might be affected as it could make some Pokémon lose trust in humans if they see such a big betrayal between two well - known trainers.
In this fanfiction, it might be that Ash has been friends with Lemon, but then Ash sides with an enemy of Lemon's for some false gain. He might think that the enemy can offer him something better, like power or wealth, and in doing so, he stabs Lemon in the back by giving away Lemon's plans or weaknesses to the enemy.
Well, it could be that Ash Aura, who was perhaps trusted by the group in the fanfiction world, reveals some dark secrets or joins the enemy side, thus betraying those who once relied on him. Maybe he was lured by power or some hidden motives.
Well, it could be that Ash makes a discovery that goes against Arceus' grand plan and is thus betrayed. Maybe he finds out something about the balance of the Pokemon world that Arceus doesn't want known.
In an Ash betrayed champion fanfiction, perhaps the champion has been secretly working with a villainous group. The champion lures Ash into a trap under the pretense of a friendly battle. Ash, being trusting, falls for it. He then has to find a way to escape and get revenge. He might meet some new allies along the way who help him see the champion's real motives.
Well, in many such fanfictions, it could involve Ash being betrayed by Clair perhaps in a Pokémon battle strategy. Maybe Clair teams up with an unexpected rival against Ash, or she might break a promise regarding Pokémon training or a joint adventure.
Perhaps May betrays Ash because she's influenced by a new group of trainers. They might convince her that Ash's way of training Pokémon is wrong or that he's holding her back from achieving greater things. So, she sides with them against Ash. This would show how easily friendships can be tested in the competitive world of Pokémon training.