In some 'harry gets betrayed fanfic', it could be Dumbledore who betrays Harry in a way. Maybe Dumbledore has been using Harry all along for a greater plan without fully disclosing it to him. Harry would feel extremely hurt and betrayed as Dumbledore was like a mentor to him. This would lead to a series of events where Harry questions everything he has known and tries to find his own path.
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 He could become more withdrawn and distrustful of others.
One possible reason could be jealousy. Maybe the Weaslys in the fanfic become jealous of Harry's fame or special abilities. Another reason might be some kind of dark magic influence on them, which changes their nature and makes them turn against Harry. It could also be due to a misunderstanding. For example, a false rumor spreads about Harry that makes the Weaslys believe he has done something terrible.
Often, the betrayal is sudden. For example, if it's his friends who betray him, it might start with a small incident that Harry doesn't think much of at first. But then it snowballs into a full - blown situation where his friends turn their backs on him completely. Then Harry has to struggle to deal with this new situation while also facing other threats in the wizarding world.
He might be initially shocked. Harry has always trusted his friends, so when the betrayal happens, it would be like a bolt from the blue. He could stand there speechless for a moment, unable to believe what has just occurred.
In the '4 kunoichi gets betrayed fanfic', it could be that the four kunoichi are working for a noble cause. However, one of their own, due to jealousy of the attention the others get, decides to betray them. She spreads false rumors about them, making the whole ninja community turn against them. The four kunoichi have to prove their innocence and also figure out a way to deal with the traitor within their group.
It could be their master. Maybe the master has been corrupted by dark forces. He sees the potential in the kunoichi and fears they will become more powerful than him. So, he betrays them to stop their progress.
Another plot could involve jealousy. If Sakura becomes jealous of Naruto's growing popularity and power among the villagers, she might betray him. She could spread false rumors about him, which would lead to the villagers turning against Naruto.
Well, the idea of a 'naruto gets betrayed by natasha fanfic' is quite an interesting crossover concept. Since Naruto is from the Naruto anime/manga world and Natasha is from the Marvel universe. It's possible that such a fanfic might be exploring the idea of how Naruto, who is used to his own set of values and relationships in his world, would react to being betrayed by Natasha. However, as I said before, actually finding it would require some in - depth searching on the appropriate fanfic platforms and communities.
In such a fanfic, the plot might start with Naruto's friends gradually turning against him due to false rumors spread by a hidden enemy. They could start to exclude him from missions and isolate him within the village. For example, Sasuke might believe the lies and challenge Naruto, thinking he has changed for the worse. This would lead to a series of battles where Naruto has to fight not only his former friends but also his own feelings of betrayal and confusion.