The wizarding community would be shocked. Guns are not part of their world, and for Harry to use one to kill Voldemort would be unthinkable for many. There would be outrage from some traditional wizards who believe in magic as the only proper way to fight.
Most of the fandom would probably be horrified. Naruto is known for his positive values, and killing a family member is completely against that. There would likely be a lot of criticism towards the author of such a fanfic.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
In 'alien first contact short stories', humans often react with a mix of fear and curiosity. For example, they might be initially terrified when they see the strange appearance of the aliens but also be extremely curious about their technology and culture.