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.
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 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.
Well, one possibility could be that he is forced to join them under some kind of dark magic threat. Maybe Voldemort has found a way to control his mind and he has no choice but to follow their orders. This would lead to a lot of internal conflict for Harry as he struggles with his own values.
The teachers at Hogwarts, like Dumbledore (if he were still alive in the fanfic), would be deeply concerned. Dumbledore always saw great potential in Harry for good. He would probably try to understand what led Harry to this path and would use all his wisdom and magic to try and turn Harry back. The other teachers would also be on high alert, fearing for the safety of the students and the future of the wizarding world.
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.
Hermione would be devastated. She has always been Harry's loyal friend and believed in the fight against the Dark Lord. She would probably try to convince Harry to come back to the right side, reasoning with him about the evil nature of the Dark Lord's actions. Ron, on the other hand, would be both angry and confused. He might initially think it's some sort of joke or that Harry has been tricked.
Harry would likely be very conflicted. He has seen a lot of death and has always fought against dark magic. The Death Note's power to kill just by writing a name is something that goes against his values. He would probably try to keep it away from others and might even consider destroying it.
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.
His relationship with Hermione would be very different. Instead of being friends, they might be enemies at first. Since he was raised by Death Eaters, he might look down on Hermione as a Muggle - born. But as he gets to know her better, he could start to question his beliefs.
One consequence could be a major shift in the wizarding world's perception of good and evil. If Harry saves the Death Eaters, people would have to re - evaluate what it means to be a 'bad' wizard. There would be a lot of distrust at first, but over time, it could lead to a more forgiving society.