Ash would likely be extremely heartbroken. He has always had a deep connection with Pokemon and believing that Arceus, a powerful and supposedly benevolent being, has betrayed him would be a huge blow. He might become withdrawn at first, not knowing who to trust anymore.
He may start to question his entire purpose. Ash has been on a journey with the goal of becoming a Pokemon master. But if Arceus has betrayed him, he might wonder if all his efforts were in vain. He could go on a quest to find out the truth about his journey, why Arceus did what it did, and whether there is still a path for him to follow in the Pokemon world that is true and not manipulated by Arceus.
Ash could also become angry. He might feel a sense of injustice and decide to confront Arceus. He would gather all his strength and the support of his closest Pokemon friends to seek answers and revenge if necessary. He has always been a fighter, and this betrayal would fuel his determination to set things right, even if it means going against the most powerful Pokemon in existence.
Ash would probably be initially shocked. He has always trusted his friends, so the idea of betrayal would be hard to swallow. He might try to talk to them, to find out why they are doing this.
Ash would feel a deep sense of betrayal that could shatter his usually upbeat attitude. He might go through a period of self - doubt. However, his love for his Pokémon and his sense of justice would pull him out of it. Mewtwo, having already experienced betrayal from humans in the past, would be filled with a cold fury. It would use its psychic abilities to gather information about the betrayers. Ash and Mewtwo would work together, Ash learning from Mewtwo's insights and Mewtwo being inspired by Ash's tenacity in the face of betrayal.
Ash could get angry. He has a strong sense of justice, and he might feel that May has violated their friendship. He could challenge her, not in a violent way, but more in a way to get an explanation and to try to make her see the error of her ways. He would likely be determined to show her that what she did was wrong.
Ash would likely be shocked at first. He's so used to seeing the good in people, especially fellow trainers. But then he'd probably get angry and determined to set things right. He has a strong sense of justice, so he wouldn't let the betrayal go unpunished.
Ash would probably be really hurt at first. He's always been so trusting of his friends. He might go off on his own for a while to clear his head, like he did when he had a falling - out with some of his friends in the past.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
Ash would probably be initially shocked. He is used to seeing the good in people and would not expect such a betrayal from Cynthia. He might stand there in disbelief for a moment, unable to process what has just happened.
There might be some new characters introduced in the fanfiction. A mysterious Pokemon trainer who has some ancient knowledge about Arceus and can guide Ash. Or perhaps a group of Pokemon researchers who have been studying the relationship between Arceus and the world for a long time. These characters could add more depth to the story, providing different perspectives on the situation between Ash and Arceus, and helping Ash in his fight against the perceived betrayal.
I think Ash would be angry as well as hurt. He's a passionate trainer and his friends mean a great deal to him. He might challenge May in a Pokémon battle, not to defeat her but to make her see how wrong her betrayal is. And during the battle, he would keep asking her to come back to their old friendship.
Ash would be extremely hurt. He's a very loyal person, and having someone he cares about like Lillie betray him would be a huge blow. He might initially go into denial, not believing that Lillie could do such a thing.