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 be extremely devastated. Pikachu is not just his Pokémon but also his closest friend. He would likely first try to reason with Pikachu, not believing that Pikachu could truly betray him. He might be in denial for a while.
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 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.
Ash would probably be very confused at first. He might try to reason with Latias, asking her why she is doing this. If she doesn't respond, he would start looking for ways to escape. He would also be worried about his Pokémon and his friends, wondering if they are safe.
Ash might be initially shocked and scared. But he's a brave trainer. He would quickly gather his courage and probably try to figure out who sent the hitman. He'd use his detective skills, which he has developed over his many adventures.
Ash would be extremely heartbroken. He might cry a lot and be in a state of shock at first. Then he would probably feel angry at the situation that led to Gary's death, whether it was a villain or just a dangerous accident. Ash would also feel a strong sense of responsibility, as if he has to carry on Gary's unfulfilled dreams.
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.