He'd probably be furious. Voldemort has a huge ego and most fanfiction would likely not portray him in the way he sees himself. He might view it as a form of disrespect to his 'true' nature and power.
She'd be appalled. Umbridge is all about following the rules and maintaining the status quo. Fanfiction doesn't adhere to the official canon she so firmly believes in.
They would probably be very curious at first. But as they read more, they might get annoyed if the fanfiction doesn't stay true to their essence. For instance, if a fanfiction portrays Sirius as a coward, which is completely against his real character, they would be offended. However, if they find a well - written fanfiction that respects their characters, they might enjoy it.
Well, some of the cast might be really intrigued. They would see how the fans have filled in the gaps or created alternative universes. However, they could also be a bit critical. If a fanfic misrepresents a character's values or relationships that were carefully crafted in the books, the actor might feel a bit disappointed. At the same time, they might also be in awe of the fans' creativity in coming up with entirely new storylines and scenarios.
He could be shocked. Maybe there are some fanfictions that take a very dark turn or have unexpected pairings that he never thought of. He'd probably need some time to process such strange ideas. And he might also be curious to see how far the creativity of the fans can go. He might even get some new ideas about his own life from reading some of the more imaginative fanfictions.
Ron would be extremely angry at first. He might feel betrayed by Harry. But as he realizes that something must be seriously wrong with Harry, he would probably join Hermione in trying to save Harry from Voldemort's influence. They would look for a way to break whatever spell or false belief has hold of Harry.
The Death Eaters would initially be overjoyed. They would think that with Harry on their side, they are invincible. But some of them might also be suspicious of Harry's motives, fearing that he is a double - agent. As for the general wizarding population, they would be terrified. Harry was seen as their hope, and now that he has joined Voldemort, they would feel completely hopeless and defenseless.
The wizarding world would be in chaos. The Order of the Phoenix would be extremely worried and likely try to rescue Harry. The Death Eaters, on the other hand, might see it as a sign of Voldemort's power and dominance. Hogwarts would also be affected, with teachers and students fearing for the future of the wizarding world.
Hermione would be horrified. She would immediately start trying to figure out a way to rescue Harry. She might start researching in the library for some ancient spells or forgotten magic that could help. Ron would be angry at Dumbledore and would probably want to rush off to save Harry without much of a plan at first.
Tony would likely laugh at some of the more out - there plot ideas. He'd probably think he could come up with better technology than what's in the fanfic. Steve would be more open - minded and might enjoy the creativity. Thor would be like 'This is not like any Midgardian story I've heard!'.
Some Avengers might be amused. For instance, Tony Stark would likely find it a fun distraction from his usual high - tech world and might even make sarcastic comments about the plot or characters. He could say something like 'These wizards should really upgrade their transportation - brooms? Seriously?'.