One possible reason for Ash getting betrayed in the fanfiction could be misinformation. A villain might spread false rumors about Ash, making his friends believe he has done something wrong. His friends, without verifying the truth, turn against him. It's a sad situation where lack of communication and trust plays a big role.
He might be initially shocked. Ash has always been surrounded by friends and supporters, so suddenly being betrayed by everyone would be a huge blow. He could be in disbelief for a while.
Another possibility is that there is some kind of dark influence at work. A powerful and malevolent force could be manipulating people's minds to turn them against Ash. This force might see Ash as a threat and use this underhanded method to get rid of him, causing his world to crumble as he is betrayed by all.
The story could end with Ash forgiving everyone. Despite the betrayal, he realizes that his friends were misled or acting out of weakness. So he takes them back and they all learn from the experience, growing stronger together.
In such a fanfiction, Ash might become extremely isolated. He could lose faith in his Pokémon and himself. Maybe he'd leave the Pokémon world in despair, feeling that all his efforts were in vain. Without the support of his friends and Pokémon, his once - strong will to be a Pokémon Master could crumble completely.
It could be a jealous rival trainer. This rival has always been in Ash's shadow and decides to plot his downfall. By spreading false information and turning Ash's friends against him, the rival hopes to finally get the recognition he thinks he deserves.
Well, often in such a 'ash gets betrayed fanfiction', the story might start with a seemingly normal day for Ash. Then, out of the blue, someone he trusts deeply, like Pikachu or one of his human friends, does something that he perceives as a betrayal. This could be something as simple as not believing in him during a crucial battle or siding with an enemy for a reason that's not clear to Ash at first. It can create a lot of emotional turmoil for Ash and make for a very engaging read.
One possibility could be that in the fanfiction, Ash betrays Latias out of a misunderstanding. Maybe he was tricked by a villain into believing that Latias was a threat to the region. This would create a lot of drama and conflict in the story as Latias, who probably trusted Ash, would be deeply hurt.
In fanfiction, a story about Ash and Latias being betrayed could be really heart - wrenching. Maybe it could involve Ash being wrongly accused by his friends and Latias being forced to turn against him due to some dark magic or false information. It might explore themes of trust and friendship being broken.