A reason could be that Itachi is framed as a traitor to the Leaf Village in the fanfiction. Naruto, being the defender of the Leaf, would then see Itachi as an enemy and take action to kill him to uphold the village's safety and honor.
In many fanfictions, the scenario of Naruto and Itachi killing the Uchiha clan is a very interesting but also controversial topic. One aspect could be the change in Naruto's character. Usually, Naruto stands for justice and protecting others. If he were involved in such a massacre, it would be a huge deviation from his original character. Maybe in the fanfiction, there could be some extreme situations that force him to do so, like being mind - controlled or having some false information.
I'm not sure specifically as there could be many interpretations in fanfiction. It might involve Itachi and Sakura in a situation where they are somehow related to the concept of 'killing your heroes'. Maybe it's about challenging the ideals or figures they once looked up to.
In some fanfictions, a possible motivation for Naruto could be to prevent a greater evil. Maybe he was tricked into believing that the Uchiha clan was about to start a war that would destroy the entire ninja world. So, he thought that by taking part in killing the clan, he could save more lives in the long run.
Fanfiction about Sakura and Itachi can be really interesting. In many such fanfictions, 'the first time' might refer to their first meeting. It could be a scenario where Sakura, with her strong will and medical ninjutsu skills, encounters Itachi, the mysterious and powerful Uchiha. This encounter could be set during a mission or by chance in Konoha. The fanfiction might explore how Sakura views Itachi's Sharingan and his brooding presence, and how Itachi perceives Sakura's determination.
The anime of Kill la Kill debuted earlier. The manga followed later, building on the popularity and story established by the anime. Generally, in this case, the anime led the way.
Their relationship could become more complex. On one hand, they have achieved their goal together, but on the other, Sasuke might feel guilty for killing his brother with Naruto's help. Naruto, in turn, might worry that Sasuke will go back to his old ways. So, while they have this new connection, there could also be new tensions and insecurities in their relationship.
Well, in the fanfiction, a likely plot development is that Naruto's newfound power, perhaps a unique form of Rasengan, is crucial in the fight against Itachi. Sasuke, on the other hand, is initially blinded by revenge but Naruto helps him see that there's more to Itachi's actions. This could lead to a situation where they don't actually kill Itachi but rather free him from whatever binds him to his 'evil' deeds. The fanfic might then explore how they deal with the consequences of not following through with the kill.
Well, start with the big fanfiction platforms. FanFiction.net is a well - known place where many fanfictions are hosted. Just use the search bar and type in the exact title 'itachi sakura to kill your heroes'. Another option is AO3. It has a great tagging system, so you can search for the relevant characters and the title keywords to narrow down your search.