Pikachu's first reaction would be to resist. It would not go down without a fight. It might start by shocking James and Meowth who are usually the ones closest to it when they try to grab it. Once in Team Rocket's custody, Pikachu would constantly be looking for a way out. It would be sad and miss Ash terribly. But it would also be brave. It could try to cause chaos within Team Rocket's base. For example, it could short - circuit any machinery they have by overloading it with electricity, which would not only be a form of resistance but also a way to create a distraction for its escape.
Pikachu would likely be very angry. It loves Ash deeply and being separated from him would make it furious. It would probably immediately start attacking Team Rocket with its electric attacks. It might also try to find a way to communicate with Ash, perhaps by using its electrical signals in a way that Ash could somehow detect from a distance.
Well, if Team Rocket finally got Pikachu, they'd probably first try to present it to Giovanni. They might be overjoyed at finally achieving their long - sought goal. But Pikachu would surely try to escape. Maybe Pikachu would use its electric powers to shock the Team Rocket members and make a run for it while they're dazed. It could also be that Ash and his friends would quickly come up with a rescue plan to get Pikachu back. They'd likely follow Team Rocket's trail and then stage a big battle to free Pikachu.
Ash would be completely devastated. Pikachu has been his closest companion since the start of his journey. He would probably go into a state of shock at first, unable to believe that Pikachu was gone.
Ash would be heartbroken. He would probably try to talk to Pikachu first, asking it what's wrong and trying to calm it down. He might be in denial at first, thinking it's some kind of joke or that Pikachu is just playing too roughly.
His friends would be very shocked at first. Misty might be a bit worried and try to find a way to turn him back quickly. Brock, on the other hand, would be more curious and start observing Ash - as - Pikachu to see how his behavior has changed. His Pokémon friends would also be confused. Pikachu himself might be both excited to have a 'Pikachu - like' friend but also a bit sad that his trainer is in this strange state.
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.
Naruto would probably be really excited. He'd treat Pikachu like a new friend and teammate. He might try to learn all about Pikachu's powers and how it can help in battles. For example, he'd be eager to see Pikachu use Thunderbolt in a fight against an enemy ninja.
His Pokémon would probably be very confused at first. Pikachu, for example, might try to zap Ash back to his senses if he starts acting like a Team Rocket member. They would likely sense that something is wrong with Ash and would do their best to get him away from Team Rocket's influence.
Pikachu would probably be very confused at first. It's used to fighting against Team Rocket, not being on their side. It might resist and try to zap Ash to snap him out of it.
The Pokémon League might condemn Ash's actions. Despite Team Rocket's wrongdoings, killing is not an acceptable solution in the Pokémon world which values friendship and co - existence. Ash could face consequences such as being banned from Pokémon battles or certain areas. On the other hand, some trainers who have been harassed by Team Rocket for a long time might secretly support Ash.
The other Pokemon in the Pokemon Center where Pikachu is usually treated would be very frightened. Nurse Joy would be at a loss as to what to do, as she has never seen Pikachu act this way. Professor Oak would be extremely concerned and would start researching immediately to find out what could have caused this sudden change in Pikachu. Team Rocket, on the other hand, might initially see this as an opportunity to capture Pikachu more easily, but they would soon realize that an evil Pikachu is far more dangerous than they expected and might end up getting in over their heads.