One possible reason could be a threat to Konoha. If Iwa planned an attack on Konoha, Naruto, as a defender of his village, might take the fight to them and end up destroying parts of Iwa in the process.
One possible reason could be a threat from Iwa to Konoha. If Iwa was planning an unprovoked attack on Konoha, Naruto might take it upon himself to destroy Iwa to protect his village.
Sasuke could be involved. He and Naruto often have intertwined stories. Sasuke might either be an ally in Naruto's destruction of Iwa, or he could be there for his own reasons, like seeking power or revenge.
There might be characters from Iwa who oppose Naruto. For example, the Tsuchikage of Iwa would surely be involved as the leader of the village Naruto is attacking.
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.
Sasuke might be able to stop Naruto. They have a long - standing relationship and Sasuke has always been Naruto's equal in terms of power. He could use his Sharingan to try and break whatever is controlling Naruto or just fight him head - on and appeal to their old friendship.
It could be an organization similar to the Akatsuki, but with a different goal. Instead of collecting tailed beasts, they are focused on conquering villages. Their army would be made up of powerful and diverse fighters, and Naruto would have to face their leader, who has some sort of special power or ability that makes them a formidable foe.
The theme of justice might be present as well. If 'kina' is something evil or causing harm in the fanfic, Naruto's destruction of it can be seen as an act of justice. Also, the fanfic could explore the theme of sacrifice. Naruto might have to sacrifice something, like his time, energy or even relationships to be able to destroy 'kina' successfully.
It could be that Naruto is possessed by an evil spirit. This spirit wants to destroy the Leaf Village for its own reasons. Naruto, under its control, would first cut off the village's supply lines, making it difficult for the villagers to get food and resources. Then, he'd start attacking the ninja in the village one by one. He might even use his Nine - Tails chakra in a very destructive way, not caring about the harm it would cause to the place he once called home.