Obviously, Harry Potter himself is the most directly affected as he doesn't survive. But then there are his friends like Hermione and Ron. They would be deeply affected, probably filled with grief and guilt. Hermione might blame herself for not being able to come up with a better plan to save Harry. Ron would be devastated, as Harry was not only his best friend but also like a brother to him.
Voldemort would be affected in a different way. Without Harry to oppose him, he would seem to have achieved his ultimate victory. But his reign might not be as stable as he thought. There could be some who would rise up against him in the name of Harry. And characters like Neville, who could potentially take on a more central role in the fight against Voldemort in Harry's absence, would also be greatly influenced.
One possibility is that the Dursleys, out of their extreme hatred for anything abnormal, actively sought ways to harm Harry. They might have found some dark magic item or spell that they used against him, and without the proper protection or knowledge to counter it, Harry succumbed. In the world of fanfiction, there are many dark and creative ways to make this tragic scenario come to pass. For instance, the Dursleys could have made a deal with some dark forces, trading Harry's life for their own 'normal' life, and those dark forces would use powerful and insidious magic to end Harry's life.
Other Hogwarts students would be affected too. Those from non - pureblood families might start to fear or resent Harry. And the pureblood students like the other Slytherins might either welcome Harry into their exclusive circle or be suspicious of his sudden change. The teachers at Hogwarts would also have to deal with this new Harry, and some might be disappointed in his transformation.
His best friends, Ron and Hermione, would probably be the most affected. They're used to seeing him with his old hair, so the change might be a shock at first.
The other students at Hogwarts would also be affected. For example, Neville, who looks up to Harry. If Harry ignores everyone, Neville might feel lost. Ginny, who has a special connection with Harry, would be deeply hurt. And the teachers like Snape, who has a complex relationship with Harry, would also be impacted. Snape might be secretly glad at first, but then realize that Harry's isolation could be a sign of a bigger problem in the wizarding world.
The Death Eaters themselves would be affected in an interesting way. Some might welcome Harry with open arms, but others might be suspicious of his sudden change of sides. They might fear that it's a trick by the Order. And within Hogwarts, the students would be in turmoil. Those who idolized Harry would be heartbroken and confused, and there could be a split among the student body depending on who believes Harry's new stance.
Obviously, Harry's closest friends like Hermione and Ron. They would be devastated. Hermione would probably be racked with guilt for not being able to prevent it. Ron would be angry and at the same time lost without his best friend.
Neville Longbottom could be a main character. He has always had the potential for great things and in a world without Harry, he would be likely to step into the spotlight. He has the courage and the connection to Hogwarts that could make him a central figure in the fight against evil.
Perhaps in this fanfiction, Harry didn't have the right items or knowledge at the crucial moment. The Elder Wand might not have recognized him as its true master, or he could have been betrayed by someone he trusted, which led to his death. Also, the magical world could have been in a more chaotic state, with darker magic being more prevalent and overwhelming Harry's defenses.
It might be that in that particular fanfiction, the author was exploring different themes such as sacrifice on a grander scale. Harry Potter sacrificing himself to save the entire wizarding world in a more permanent way than in the original story. For example, perhaps the threat in the fanfiction was so great that his death was seen as the only solution to end the chaos and darkness.
The Weasley family is also likely to be among those affected. Given their large family and close - knit nature, the death of one of them could have a ripple effect on the others. For instance, Fred's death in the original series had a profound impact, and in fanfiction with multiple deaths, more of the Weasleys could be in danger.