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.
It could be his teachers at Hogwarts. For example, Snape might finally turn on Harry completely in this fanfic. Snape could reveal some secrets that make Harry feel like he's been lied to all his life. This betrayal by someone he thought was an ally could push Harry towards Voldemort.
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 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.
Perhaps he was deceived. There might be a powerful dark wizard who managed to convince Harry that the dark side had a more 'just' cause. This wizard could have used false information about the history of the wizarding world and how the dark side was initially misjudged.
It could be Ron. Maybe he starts to feel that Harry has changed too much after the war and is no longer the friend he once knew. He might be influenced by outside forces or his own insecurities. Another possibility is Hermione. If she gets too involved in the new Ministry politics and starts to believe that Harry's actions are not in line with the new wizarding order, she could turn on 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.
In 'harry potter is betrayed fanfic', Luna Lovegood might be a character who betrays Harry. Maybe she is under the control of some mind - altering spell or has been deceived by a false prophecy that makes her think she is doing the right thing by betraying him. Also, Hagrid could be a shock betrayer if he is somehow forced to choose between Harry and his beloved magical creatures, and chooses the latter in a way that is seen as a betrayal to Harry.
Snape could play a crucial role. Given his past with both Voldemort and his role at Hogwarts, he might be torn between his loyalty to Dumbledore's cause and his complex relationship with Harry. He could be the one who has to make a difficult decision when Harry starts to side with Voldemort. Additionally, Draco Malfoy could be involved. He might see Harry's change as an opportunity to gain more power within Voldemort's ranks, or he could be jealous of Harry's new - found position with the Dark Lord.
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.
Even Ginny could be written as a betrayer in a fanfic. For example, if she becomes possessed by a dark force, she might turn against Harry. Her relationship with Harry is so central in the original story that a betrayal from her would be a huge shock and create a lot of drama in the fanfic.