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.
It could be that his parents were trying to protect Harry in a really messed - up way. They might have thought that by 'betraying' him, like sending him away or making him seem like an outcast, they could keep him safe from a much greater threat. Maybe they knew some prophecy about Harry and thought this was the only way to ensure his survival. They might have made a deal with some dark force, believing they could outwit it in the end but ended up being seen as traitors by Harry and others.
In such a fanfiction, perhaps the parents were forced to betray Harry due to some dark magic or a powerful enemy threatening them. Maybe they had to give up Harry to save the wizarding world from an even greater evil.
Maybe they were forced into it by some dark magic or a powerful dark wizard. They could have been blackmailed into betraying Harry to save their own lives or the lives of others they cared about.
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.
He would be extremely confused. Harry has always thought of his parents as heroes, and suddenly finding out they betrayed him would shatter his world view. He'd question everything he believed in.
In some fanfiction, the idea of Harry Potter being betrayed by his parents might be justified through a complex web of magic and prophecy. Maybe the parents thought sacrificing Harry was the only way to save the wizarding world from a greater evil. However, this goes against the essence of their characters in the original books.
One possible reason could be that in the fanfic, the author might twist the story to make it seem like his parents' actions had unforeseen consequences that affected Harry negatively. For example, their fight against Voldemort led to Harry being an orphan and having to bear a great burden from a young age.