Well, it could be a really dark take on the story. Perhaps the author of the fanfiction wanted to explore a 'what if' scenario. Maybe Ginny was jealous of Harry's fame or felt overshadowed by him. But in the canon, Ginny was always a loyal and loving partner to Harry, so it's a very different and somewhat untrue - to - the - spirit - of - the - original view in the fanfiction.
Fanfiction often takes creative liberties. For Ginny to betray Harry in a fanfic, it might be part of a complex plot where she is tricked into believing Harry has done something wrong. For example, an enemy could have fabricated evidence against Harry. But this is a far stretch from the Ginny we know from the books, who had deep love and respect for Harry. It shows how fanfiction can turn the known characters and their relationships on their heads for the sake of exploring new and often controversial ideas.
It could be due to jealousy. Perhaps Ginny has been spending a lot of time with another character, and Harry misinterprets their relationship, leading to his anger. In fanfiction, the writers often play with such emotions to create drama.
One possible plotline could be that Harry is so focused on his fight against Voldemort that he doesn't have the emotional space to develop feelings for Ginny. He might see her as just a friend or even as a younger sister figure. For example, he could be constantly training, researching dark magic defenses, and leading the Dumbledore's Army, leaving no room for romantic thoughts about Ginny. Another aspect could be that Harry has unresolved trauma from his past, like the death of his parents, which makes him hesitant to get involved in a romantic relationship with Ginny.
One possible plotline could be that Harry betrays Draco in a duel. Maybe there was some sort of misunderstanding or a secret deal that Harry made with someone else that required him to turn on Draco during the duel. For example, if there was a dark wizard threatening Harry's friends and the only way to save them was to sacrifice Draco in the duel by using a forbidden spell against him.
Well, in a fanfiction context, it could be possible if we consider a crossover universe where Naruto somehow ends up in the Star Wars galaxy. Maybe he's brainwashed by the dark side's influence which is similar to some dark forces in his own world. This could lead him to betray the Jedi ideals that he might have adopted during his time there.
In some fanfictions, Ginny might be written to betray Harry because of a prophecy. The prophecy could mislead her into thinking that Harry was going to bring doom, and so she decides to act against him to prevent that. It's a different way of looking at their relationship compared to the original story, where their bond was based on love and trust.
One possible reason could be a misunderstanding. Maybe Ginny is under the influence of a powerful dark magic that makes her act against Harry without her full control. For example, a powerful mind - controlling spell could be involved.
Perhaps Harry discovers a dark secret about the side he has been fighting for. This discovery makes him feel betrayed and he turns to Tom thinking that Tom offers a different kind of solution. It could be that Tom presents himself as a reformed person or has a convincing argument about a new order in the wizarding world that Harry, in his disillusioned state, starts to believe. Then, the story could follow how Harry's relationships with his old friends deteriorate as he gets closer to Tom's side.
She might hate it because it could change the way people see her character. In the original story, she is a strong and independent person, but fanfiction might portray her in a wrong way.
Ginny would most likely be extremely happy. She has deep feelings for Harry, so his proposal would be a dream come true for her.
A great deal of fanfiction delves into their love. You can find ones that show Ginny being the strong - willed and supportive partner to Harry. For example, in some stories, Ginny helps Harry deal with his PTSD from the war by using her understanding of magic and her love for him. Their love in fanfiction often represents a combination of passion and loyalty.