Pikachu would probably be very curious. It might zap the lightsaber a bit to see what would happen, and then be in awe of the bright blade. It could also get a bit protective of Ash when he wields the lightsaber, as if it senses the added danger.
However, as time goes by, Ash could come to understand that Mewtwo's overprotectiveness comes from a place of care. He might start to rely on Mewtwo in certain situations, like when they are in an extremely dangerous and unfamiliar territory. Ash could also try to communicate with Mewtwo, asking it to let him handle some things on his own while still appreciating its protection.
In some fanfictions, Ash gets angry. He might be angry at the world, at fate, for taking his mom away. This anger could manifest in different ways, like being more reckless in battles or snapping at his friends when they try to comfort him. But deep down, it's just his way of dealing with the overwhelming pain of losing his mother.
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 be deeply hurt. He's a trusting person, so he'd probably feel a mix of anger and sadness. He might initially be in shock, not believing that Ursula could do such a thing.
Ash would initially be heartbroken. He has shared so many adventures with May, and this betrayal would shatter his trust. He might go off on his own for a while to think things through. When he comes back, he could be more determined than ever to prove himself without May. He might also be a bit more cautious when it comes to making new friends. His Pokémon would sense his distress and try to comfort him, but it would take a long time for him to fully get over May's betrayal.
Well, they might feel a range of emotions. Some could be angry, feeling that they were wronged after putting their trust in Ash. Others might be heartbroken, especially if they had deeper feelings for him. For example, one of the harem members who was really close to Ash might go into a state of denial at first, not wanting to believe that he could do such a thing.
Ash might initially be confused. He's used to his mother's love, but this new level of possessiveness could stump him. He may not understand why she's suddenly so overbearing.