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.
There's also a possibility that it's a misunderstood Pokémon. This Pokémon has some sort of grudge against Ash, perhaps thinking Ash accidentally wronged it in the past. So, it uses its powers to make everyone around Ash believe he's a bad person, thus causing the mass betrayal.
Maybe it's an evil organization that wants to use Ash's popularity against him. They figure that if they can make everyone turn on Ash, they can create chaos in the Pokémon world. They start by brainwashing some of Ash's closest friends to start the chain of betrayal.
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.
It could be the very villagers who once looked up to Naruto. Their betrayal could turn them into antagonists as they might try to further harm Naruto to protect their own interests.
Ash himself could be the main antagonist as he is the one doing the betraying.
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.
The main antagonist might be an organization that wants to take over the Hidden Leaf Village. They see Naruto as a major obstacle because of his strength and influence among the villagers. So, they devise a plan to make everyone in the village betray Naruto. They use mind - control techniques or spread misinformation on a large scale to achieve their goal. This way, they can weaken the village from within before making their move to take it over.
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.
Maybe an ancient evil spirit that has a vendetta against Ash. The spirit could possess the aura guardian and make it turn against Ash. In this scenario, the main antagonist is not really the guardian but the spirit that controls it, and the story would be about Ash fighting this spiritual evil.
It could be a powerful and jealous Galar trainer. This trainer might see Ash as a threat to their own reputation and success in the region. They would then use their influence and charm to turn Ash's friends against him.