One popular 'ash becomes evil after betrayal fanfiction' is 'Ash's Dark Descent' which explores how Ash's trust is shattered and he turns to the dark side. It has some really intense scenes where you can see his transformation.
In some fanfictions, Ash might become evil after betrayal when his trust is shattered. For example, if his friends turn on him for some false accusations, he could start to see the world in a different, more cynical way. This broken trust might lead him to believe that power is the only thing that matters, and he starts to pursue it without regard for the consequences, gradually turning evil.
A typical sign is his disregard for the well - being of others. Previously, Ash might have been a selfless character, but after betrayal, he may start to use people for his own gain. Also, his appearance might change in the fanfiction. He could start to dress in darker colors or have a more menacing look in his eyes, symbolizing his inner transformation.
Gary could be the main antagonist. He has always been Ash's rival and seeing Ash turn evil, he might feel responsible to stop him. Gary could gather his own team of powerful Pokémon and use his wits to oppose Ash at every turn.
One possible plotline could be that Ash is betrayed by his friends. This heartbreak and sense of abandonment turns him evil. He then starts using his Pokémon in cruel ways to seek revenge on those who wronged him.
Power and growth are also typical themes. Having Reshiram gives Ash new power, and he has to grow in order to handle it properly. The fanfics usually show how he learns to control Reshiram's power and how it affects his battles. For instance, he might face stronger opponents but with Reshiram, he has new strategies to overcome them. Also, the theme of trust is important. After the betrayal, Ash has to learn to trust again, this time with Reshiram, and the story often explores how this trust develops over time.
There's also a fanfic series that delves into this concept on Archive of Our Own. In it, Ash's betrayal is a turning point. Reshiram comes to him as a symbol of new hope. The story is really detailed in exploring Ash's emotions and his new relationship with Reshiram. For example, it shows how he trains with Reshiram and how they face various challenges together. It's a great read for fans of this kind of scenario.
In some fanfictions, Ash might become evil after heartbreak by losing his faith in the goodness of the world. For example, if a character he deeply trusted betrays him, he could start to view relationships as tools for power or revenge. This new mindset might lead him to make choices that are considered evil, like using underhanded tactics to defeat opponents or turning a blind eye to the suffering of others.
Their relationship would likely become more guarded. They might start to question their judgment in trusting others. Ash and Brock would become more reliant on each other than ever before, knowing that they are the only ones they can truly trust at that moment. For example, they might start double - checking every new acquaintance together to avoid another betrayal.