In my opinion, it's not very ethical. Naruto, Hinata, and Sakura are well - loved characters. Writing bashing fanfiction can be disrespectful to the original creators and the fans who adore these characters. It can also create a negative atmosphere within the fan community.
One common theme could be focusing on Naruto's relationship with other characters aside from Hinata and Sakura. Maybe exploring his bonds with characters like Sasuke or Kakashi more deeply in a way that downplays Hinata and Sakura's significance in his life.
Some people might write it because they prefer other characters over Naruto, Hinata, and Sakura. They could be trying to promote their favorite characters by putting these three down in their fanfiction.
Well, in this type of fanfiction, a popular trope is rewriting the story so that Naruto realizes his love for Hinata much earlier and completely ignores Sakura. This can involve creating scenarios where Sakura tries to interfere in their relationship but fails miserably. Also, some fanfics might bash Sakura by making her actions seem more selfish, like her pursuit of Sasuke being portrayed as just for her own vanity rather than true love, while Naruto and Hinata are shown as having a pure and selfless love.
It could also be that they have a different vision for Naruto's character arc. They might think that Hinata and Sakura held Naruto back in some ways in the original story, so they create fanfiction to show what they believe Naruto could have achieved without their influence. Additionally, some might just be expressing their dislike for Hinata and Sakura's personalities as they were portrayed in the official story.
I don't really encourage 'bashing' fanfics as they are often mean - spirited. However, if you're just curious, you might search on some niche fanfic websites. But remember, Naruto and Hinata, and Sakura are all well - developed characters in the original series, and bashing them in fanfics goes against the spirit of the source material in a way.
Some fans might write it because they're big fans of the Naruto - Hinata pairing. They feel that in the original story, Sakura got in the way of their relationship development, so they write fanfics to change that. They want to see more of Naruto and Hinata's love story without Sakura's interference.
In fanfictions that involve 'Naruto good Sasuke Sakura bashing', often the author wants to create an alternate storyline. Naruto's good nature might be magnified, like showing him as an even more selfless hero. But when it comes to bashing Sakura, it might involve writing her as a weak - willed character who makes bad decisions. And for Sasuke, they could rewrite his story to make Naruto seem more important in his life and development, perhaps by downplaying Sasuke's own growth and making it more dependent on Naruto's influence.
Some people might write them out of jealousy. For example, if they prefer other characters and feel that Naruto, Hinata, or Sakura get too much attention in the original series, they might write bashing fanfics. It's a rather immature way to deal with their feelings though.
One possible reason could be for the sake of creating a more complex love triangle in fanfiction. Maybe the writer wants to explore a different dynamic where Naruto and Hinata's relationship is strengthened by having a common dislike for Sakura. It could also be that the fanfiction writer is trying to give Hinata more agency and make her a stronger character by having her oppose Sakura in some way, perhaps in matters related to Naruto's affections.