Yes, it is considered controversial. Harry Potter is traditionally seen as the hero who opposes Voldemort, the ultimate villain. The idea of him joining Voldemort goes against the core values of the original story. Fans of the original series often have strong emotional attachments to the characters as they are in the books, and this concept can be seen as a perversion of those characters.
Perhaps Harry joins Voldemort out of a sense of hopelessness. After seeing so much destruction in the wizarding world, he might think that joining Voldemort is the only way to end the war quickly. However, he soon realizes that Voldemort's methods are too extreme and tries to find a way to turn things around from within the dark side.
In these fanfictions, Harry's character transformation is quite drastic. He loses his sense of morality. His loyalty, which was once towards his friends and the good side, now shifts to Voldemort. He becomes more power - hungry, willing to do whatever it takes to gain more power, including using dark magic without hesitation. He also becomes more secretive and manipulative, using his former relationships to further Voldemort's goals.
In this fanfiction scenario, it could be that Voldemort was misunderstood due to a long - standing feud between two powerful wizarding families. The real motives of Voldemort were to bring equality among all wizards, regardless of their blood status. Harry, who has always fought for justice, sees the potential in this 'good' Voldemort and decides to join him. They then start a journey to change the perception of the wizarding society towards Voldemort and his ideas.
In some versions of this type of fanfiction, there could be a time - travel element. Harry travels back in time and ends up getting involved with a younger Voldemort. He sees potential in Voldemort that he believes can be redirected for good, but instead, he gets drawn into Voldemort's dark plans. As a result, he begins to work with Voldemort against the very people he once called friends and allies.
In such fanfiction, a common plotline is that Harry's decision to join Voldemort creates a power shift. He might use his status as the 'Boy Who Lived' to undermine Dumbledore's reputation further. Since many in the wizarding world had put their faith in Harry, his joining Voldemort could lead to widespread panic and confusion. Harry could also start to recruit other disillusioned wizards to Voldemort's cause, using his own charm and the false narrative that Voldemort is actually a liberator of the wizarding world from the old, restrictive ways.
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.
Well, in a fanfic where Harry joins Voldemort, it could be a very different story. Harry could start to learn the dark arts from Voldemort, which would change his character completely. He might be given new powers or abilities that are dark in nature. This would also cause a lot of confusion among the wizarding community. People who once looked up to Harry would be disappointed and afraid. And there would be a new dynamic between the Death Eaters and Harry, with some perhaps being jealous of the attention he gets from Voldemort while others might try to befriend him for their own gain.
In a 'harry joins voldemort fanfiction', perhaps Harry discovers a dark secret about the wizarding world that makes him think Voldemort was right all along. It could be something about the Ministry of Magic being corrupt and Voldemort's way of ruling, although extreme, is the only solution. So he joins Voldemort to 'fix' the wizarding world.
In a fanfic with this scenario, the story would take a dark and twisted path. Harry joining Voldemort would mean that the values he once held dear are now being tossed aside. Voldemort would start to manipulate Harry, perhaps using memories of his parents' death to make him more bitter. The Order of the Phoenix would have to reevaluate their strategies as Harry now knows a great deal about them. And as Harry gets more involved with Voldemort's plans, he might start to see the world in a more cynical way. His magic could also change, becoming darker and more unpredictable as he delves into the dark arts under Voldemort's tutelage. This would lead to a series of events where the line between good and evil becomes blurred for Harry and for the readers.