Maybe in this fanfiction, Harry gets tired of always being the 'good guy' and having to bear so much responsibility. He sees Voldemort as someone who has power without the constraints that he has. Voldemort tempts Harry with the idea of freedom from the expectations of others, and Harry succumbs and joins him.
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.
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.
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.
Perhaps Harry joins Voldemort because he believes Voldemort's lies about the wizarding world. Voldemort could tell Harry that the Order of the Phoenix is actually trying to use him and that he would be better off with him. In this scenario, Hogwarts would become a very different place. Teachers would be in a state of panic, not knowing how to deal with Harry's new allegiance. The other students would be terrified, and some might even be swayed to Voldemort's side as well, seeing Harry as a role model. It would be a chaotic and dark situation.
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 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.
One interesting plot twist could be that Fem Harry's presence among Voldemort's ranks causes internal conflict. Some Death Eaters might be distrustful of her because she was once the 'Boy Who Lived' in a female form. Another twist could be that Fem Harry starts to influence Voldemort's ideology in a way he didn't expect. Maybe she makes him question his pure - blood supremacy by showing that her Muggle - born friends were as powerful as any pure - blood wizard. Also, it could be that her joining was part of a long - con plan to bring down Voldemort from the inside, and she gradually starts to turn his own followers against him.
Well, it could be that her joining Voldemort causes a huge shift in the balance of power. The wizarding community would be in chaos as they try to figure out how to deal with this new threat. Dark Fem Harry might also start to influence Voldemort in unexpected ways. Maybe she convinces him to use more psychological warfare rather than just brute force. And she could be the one to find and destroy some of the remaining Horcruxes of other wizards, believing that it would make Voldemort even more powerful and thus her position more secure.
The main antagonist could be Dumbledore. Since Harry has joined Voldemort, Dumbledore would see Harry as a threat to the good of the wizarding world. Dumbledore would try to bring Harry back to the right side, and they would end up in conflict.