In some fanfiction, it could be a mind - control situation. Voldemort manages to gain some sort of control over Harry's mind, forcing him to side with him. But Harry's friends and his inner strength gradually break through the control, and he then has to fight not only against Voldemort but also the stigma of having been on the wrong side.
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 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 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.
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 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.
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.
Well, in such a fanfiction, Harry Potter might bring a new perspective to the Hellsing crew. He comes from a world where magic has different rules and ethics. His presence could lead to some internal conflicts within Hellsing as they debate whether to trust his 'alien' magic. Also, Harry might be shocked at the extreme measures Hellsing takes against vampires and other supernatural beings, and try to find a more 'humane' solution while still protecting humanity.
In this fanfiction, Harry could be brought into the Unspeakables to deal with some ancient and powerful magic that has been awakened. He may start off as an outsider among the Unspeakables, but gradually earn their respect. His adventures within the Department of Mysteries could include uncovering long - lost spells and artifacts. This could also lead to a conflict between his loyalty to his old friends and his new responsibilities within the Unspeakables.
Well, one possibility is that Harry could use his magic skills in a discreet way to help the BAU team. For example, he could use a spell to make a suspect more talkative during an interrogation. Another could be that his experience with the dark arts and fighting Voldemort gives him an edge in understanding the motives of really evil criminals that the BAU might be hunting. He might also introduce the team to magical creatures that could be used as analogies for certain criminal behaviors.