In this fanfic, Harry might be betrayed by the Order of the Phoenix. They could have made a decision that endangered Harry's life without his knowledge. Feeling used and discarded, Harry runs away. Voldemort, through his Death Eaters, finds Harry and manipulates him. He tells Harry that he can give him a place where he will be respected and not betrayed again. Harry, still young and naive in some ways, falls for Voldemort's lies and joins him, leading to a very different kind of war in the wizarding world.
Perhaps Harry is betrayed by the wizarding world as a whole. The Ministry of Magic wrongly accuses him of a serious crime. With no one to turn to, Voldemort approaches Harry. Voldemort shows Harry a world where he can be in control, instead of being at the mercy of those who have turned against him. So, Harry decides to join Voldemort out of desperation and a thirst for revenge against those who betrayed him.
In the 'harry joins voldemort fanfic', it could start with Harry being captured by Voldemort. Instead of killing him immediately, Voldemort decides to brainwash him. Through a series of dark magic spells and psychological manipulation, Harry starts to see Voldemort as a mentor. As time passes, Harry becomes an integral part of Voldemort's inner circle, helping him plan attacks against the wizarding world, believing that they are actually for a greater good as Voldemort has convinced him.
The Ministry of Magic could be the ones to betray Harry. They might try to use Harry as a pawn in their political games, and when Harry realizes this, he loses faith in the so - called good side. Voldemort then takes advantage of Harry's disillusionment and convinces him to join.
Hermione could be the one. Maybe she gets jealous of Harry's fame and power, and starts spreading lies about him, which leads to his betrayal. As a result, Harry feels so hurt that he turns to Voldemort.
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.
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.
One possible plot could be that Harry is influenced by Voldemort's false promises. Maybe Voldemort offers to bring back Harry's parents in exchange for his loyalty. Harry, in a moment of weakness and longing, decides to join him. However, once he realizes Voldemort's true nature of deception and destruction, he has to find a way to break free from this dark alliance.
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.
Well, it could be that Evil Harry is actually a long - lost relative of Voldemort in this fanfiction. He discovers this connection and, feeling a sense of kinship with Voldemort's ideology of pure - blood supremacy, decides to join him. He then uses his skills at Hogwarts, like his knowledge of spells and potions, to enhance Voldemort's army. He might also be involved in turning some of his classmates to the dark side, creating a rift within Hogwarts between those who follow him and those who oppose Voldemort's new ally.
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.