One common theme could be exploring alternative storylines where the absence of Sasuke and Sakura leads to different power dynamics in the Naruto universe. For example, other characters might have to step up and fill the void left by Sasuke's power. Another theme could be a focus on Naruto's reaction and how he copes with the loss of his friends, which could lead to character development in ways not seen in the original series.
Some fans write it because they want to explore different plot directions. They might not be satisfied with the original story's development regarding Sasuke and Sakura, so they create a scenario where they're dead to see how the story could change. It gives them creative freedom.
Yes, there might be such fanfics out there. In the Naruto fandom, with a large number of fans creating various types of stories, it's possible that some would write a story where Naruto destroys Sasuke in a way that could be seen as Sasuke - bashing. However, these kinds of fanfics may not be in line with the overall spirit of the original work which focuses on their complex relationship and growth.