A new, powerful and mysterious dark wizard or witch could be the main antagonist. This character could have some hidden agenda that involves breaking Harry's support system. They could target his friends, use their weaknesses, and make them betray Harry for their own gain or to further the dark wizard's evil plans.
The Ministry of Magic could also be the main antagonist in this case. If the Ministry is corrupted or under the influence of dark forces, they could manipulate Harry's friends. They might convince his friends that Harry is a danger to the wizarding world order, and his friends, being naive or misinformed, end up betraying him.
In such a fanfiction, perhaps Harry's friends are jealous of his newfound powers or popularity within the wizarding world. So they spread rumors about him being untrustworthy. As a result, Harry is shunned by many in the community. He has to embark on a solo journey to prove his innocence and in the process discovers who his true friends really are.
Well, usually in such fanfictions, Harry might face a lot of emotional turmoil and have to find his way alone to overcome the betrayal and the challenges that come with it.
It could be that an external force, like a dark wizard in disguise, is the main antagonist. This dark wizard targets Hermione because of her potential to be a great threat to their evil plans. They use magic to cloud the minds of her friends, making them believe Hermione is up to no good. The friends, being under this spell, betray Hermione without realizing they are being manipulated. The dark wizard then watches as Hermione struggles with this betrayal, all part of their grand scheme.
In some fanfictions, it could be that Harry discovers a dark secret about the wizarding world that his friends don't believe or are too afraid to accept. His friends, thinking he has gone mad, side with the established order. They then betray Harry by reporting his so - called 'crazy ideas' to the Ministry of Magic, leading to Harry being hunted.
It could be Draco Malfoy. Since Harry and Ron are not friends, Draco could take advantage of their weakened state. He might try to turn them against each other even more, perhaps by spreading false information about one to the other. He could also try to isolate Harry from the Gryffindor group since Ron is no longer on his side, making Harry an easier target for his bullying.
Well, one reason could be jealousy. Maybe his friends envy his fame or special abilities. Another could be misinformation. If they are fed false information about Harry, they might turn against him. For example, if someone convinces them that Harry is secretly working for the enemy.
At first, Harry would be in disbelief. He would think it was some sort of joke or misunderstanding. But as the reality sets in, he would become a bit of a loner. He'd still have his sense of right and wrong, so he might continue to fight against the dark forces on his own, all the while hoping that his friends would come to their senses and realize they made a huge mistake in betraying him.
In some fanfictions, Harry could be betrayed by friends due to jealousy. For example, if a friend is jealous of Harry's fame as the 'Chosen One'. Another reason might be a misunderstanding. Maybe Harry is wrongly suspected of using dark magic, and his friends, without proper investigation, turn against him.
In a fanfiction where Naruto is betrayed by his friends, it could start with jealousy. Suppose Naruto gets a great power or a special recognition. His friends, feeling overshadowed, might be tricked by an evil force into thinking Naruto is using them. So they betray him, which makes Naruto realize that he has to rely on his own strength and find new allies who truly understand him.
Sometimes in fanfictions, character development takes a dark turn. The friends might have their own personal goals that conflict with Naruto's. They could see Naruto as an obstacle to achieving those goals. Or perhaps there was a long - hidden secret that came to light, which made them change their attitude towards Naruto, leading to the so - called betrayal.