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.
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.
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.
His reaction would depend on how the betrayal was presented. If it was a sudden revelation, he would be shocked and devastated. He may start to question his own worth, thinking that if his own parents could betray him, what was wrong with him? He could also become more reckless in his actions, not caring about the consequences as much since he felt like he had already lost the most important thing. This new Naruto could also be more empathetic towards the villains in the story, understanding their feelings of being wronged or betrayed in some way.
Perhaps his parents were part of a secret and evil organization all along. They pretended to be good but in fact were planning something big and Harry was just a pawn in their plan. So they betrayed him by leaving him exposed to danger or even trying to harm him directly. In the fanfiction, Harry could then go on a journey to discover the truth about his parents and their real motives.
If Harry was betrayed by his parents in fanfiction, he would likely become a much more cynical and distrustful person. His whole world view was based on his parents being heroes, so this betrayal would shatter his beliefs and make him question everything.
In some fanfictions, it could be due to a long - hidden family secret. His parents might have had a past that they were ashamed of or a debt they had to pay, and unfortunately, Harry became a part of that repayment which led to the perception of betrayal. They could have made a deal with a dark force before Harry was born, not realizing the full implications it would have on their son.