It could be Orochimaru. He has always been a source of trouble and if he manages to pull off some really devious plan that directly affects Naruto or his loved ones, it might cause Naruto to snap.
Well, it could be that Naruto finally snaps due to the continuous betrayal or loss he has faced. He might turn against his former friends or village. For example, if he discovers some dark secrets about the village that have been hidden from him all along, this could be the final straw. He could then start his own journey, not as a hero of the village but as a sort of anti - hero, challenging the existing order.
In this kind of fanfic, Naruto might finally lose his temper completely. Maybe he could turn against his friends or the village. He could start using his power in a more unrestrained way.
In a 'naruto finally snaps fanfic', Naruto is the main focus, but characters like Kakashi would be involved. Kakashi has been Naruto's teacher and mentor, so he would be affected by Naruto's change. Also, characters from other villages such as Gaara could be part of the story. Gaara has his own connection with Naruto, and Naruto's snapping would have an impact on their relationship and the balance between villages.
Well, it could be that Hinata has been pushed too far by the constant bullying or pressure in the Naruto world. Maybe she finally snaps at those who have underestimated her all along, like some of the meaner villagers or perhaps even some of her rivals during a particularly intense moment in a fight or competition.
Well, it might be that Harry snaps due to his ongoing struggle with Voldemort. After constantly being in a state of fear and having to fight for his life and the lives of his friends, he reaches his limit. He could turn against some of his friends who he feels haven't been fully supportive or understanding. His snapping could lead to a power shift in the wizarding world, as he may start to use his magic in more aggressive and unorthodox ways to protect himself and what he believes in.
Well, it could be about Tsuna reaching his breaking point after constantly facing difficult situations, like too many battles or personal betrayals. Maybe he snaps and decides to take a completely different approach to things, perhaps becoming more aggressive or even distancing himself from his friends for a while.
Obviously Tsuna would be the main character. His friends like Gokudera and Yamamoto are likely to be in it too. They might be the ones who are shocked by Tsuna's snapping and try to help him get back to normal. Reborn could also be there as he has been guiding Tsuna all along and would surely play a role in this situation, either trying to understand what made Tsuna snap or trying to fix it.
Well, it could be about Harry finally losing his patience with all the constant threats and dark magic. Maybe he decides to take a more aggressive approach against Voldemort and his Death Eaters, instead of always waiting for them to make the first move.
It could be Neji. Even though they are family, Neji has been quite tough on Hinata in the past. His overbearing nature and the way he has sometimes looked down on Hinata could be the final straw that makes her snap.