One possible reason for Harry being betrayed in a second - year fanfic could be a miscommunication. Maybe a new character misunderstands Harry's intentions and spreads false rumors, making others think he has betrayed them. For example, if Harry is trying to protect a secret about Hogwarts but his actions are misconstrued.
He could be initially shocked. Harry, who has trusted Ginny deeply, would find it hard to believe at first. He might go through a period of denial, trying to find excuses for Ginny's actions.
At the start, Harry would be stunned. His whole life has been built on the idea of his parents being heroes. When he realizes they betrayed him, he would be heartbroken. He might go on a journey to find out the real motives behind their actions. Along the way, he could become bitter and vengeful. However, he might also find some inner strength and decide to rise above the betrayal. He could use it as a motivation to break free from the shadows of his parents and create his own path, but not without a great deal of emotional turmoil first.
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.
One common theme is isolation. When Harry is betrayed, he often finds himself alone, cut off from his previous support systems. For example, in many fanfics, he has to leave Hogwarts or avoid his friends. Another theme is revenge. Sometimes Harry seeks revenge on those who betrayed him, which adds a darker element to the story.
Well, it could be many reasons. Maybe there was a misunderstanding. Some dark magic might have been used to make his friends think he did something bad. Or perhaps a powerful enemy framed him by planting false evidence. It's hard to say exactly without more context from the fanfiction.
Harry might be initially shocked and heartbroken. He trusted the Weaslys like family, so the betrayal would hit him hard. He could become more withdrawn and distrustful of others.
He might be extremely angry and confused. After all, parents are supposed to be the ones you can trust the most. He could start to question everything he knew about his family and himself.
In a 'Harry Potter betrayed fanfic', perhaps Hermione turns against Harry. It could be because she disagrees with Harry's more dangerous and reckless plans to fight Voldemort. She might think he's endangering too many lives without proper thought, and this leads to a rift and a sense of betrayal.
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.