The wizarding world would be in shock. Harry was seen as the 'Chosen One' to defeat Voldemort. If he joined the Death Eaters, the Order of the Phoenix would feel betrayed. They might try to rescue him or think he's been an undercover agent all along. Hogwarts would be in chaos, and students would be confused and scared.
The Hogwarts students would be divided. Some would be in disbelief and would still hope that Harry has a good reason for joining. They might secretly support him. Others, especially those who have lost family members to the Death Eaters, would be furious and see Harry as a traitor. Harry's old enemies at Hogwarts, like Draco Malfoy, would also be confused. Draco might initially think that Harry has finally seen the 'right' side, but as he realizes Harry's true intentions (if they are good), he would be conflicted as well since he's been brought up in a Death Eater - influenced family.
The Order of the Phoenix would see Harry's joining of the Death Eaters as a major setback. They would likely view it as a sign that Voldemort has become even more powerful. Dumbledore, if he were still alive, would be very concerned and would try to understand what led Harry to this decision. He would also be devising a plan to get Harry out of the clutches of the Death Eaters.
There would likely be a great deal of anger and a thirst for revenge. Harry has many friends and supporters. They would probably form some sort of resistance against Voldemort, but they would also be mourning Harry's death deeply. Some might even question if there was more they could have done to prevent it. The Ministry of Magic would also be in a state of turmoil, as they had been relying on Harry to end the threat of Voldemort.
Well, if we look at 'Harry Joins Death Eaters Fanfic', it could lead to a whole new set of storylines. Harry might be lured in by the power the Death Eaters seem to have. He could start using dark magic, and his moral compass would be all messed up. Maybe he'd be used by Voldemort to infiltrate the Order of the Phoenix from the inside. It would be a very different Harry, one who has lost his way and is now on the side of evil.
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 shock. Harry has always been seen as a symbol of hope and the one who could defeat Voldemort (Tom). People would feel betrayed, especially his friends like Hermione and Ron. The Ministry of Magic would likely see Harry as a new threat and start devising plans to stop him. There would be a lot of fear and uncertainty among the wizarding population.
The wizarding world would be in shock. People who once saw Harry as their savior would feel betrayed. The Order of the Phoenix would be in disarray, not knowing whether to try to rescue Harry or consider him an enemy. Hogwarts would also be in turmoil, with students and teachers alike not believing that Harry could turn to the dark side.
Their view could become more confused. Fanfiction can present all sorts of alternate universes and character developments. The Death Eaters, used to a very specific view of the wizarding world under Voldemort, would find it hard to accept different versions. They might see the wizarding world in fanfiction as a chaotic and wrong place, which would make them double down on their own views to feel more secure.
Some Death Eaters might be angry. They have a certain self - image and ideology in the original story, and if fanfic distorts that in a way they don't like, they would not be happy. For instance, if a fanfic makes them out to be weak or cowardly, they would likely react negatively. However, others might be more open - minded and see it as an interesting alternative view of their world, but this would be a minority among them, as they are generally quite set in their ways.
The wizarding world would be in shock. Harry is seen as their last hope, so his surrender would make many lose faith. The Order of the Phoenix might go into a state of chaos, trying to figure out how to rescue Harry and continue the fight against Voldemort without him.