Hinata would be in shock. She'd probably freeze for a moment, unable to believe what she's seeing. Her eyes would be wide open in horror.
She would then likely break down in tears. All her love for Naruto would come pouring out in her grief. She might try to hold him, begging him not to leave her.
She could also get angry. Angry at the situation, angry at whoever or whatever caused Naruto's death. This anger might give her a sudden burst of strength, but it would be short - lived as the grief would soon overtake her again.
He would be filled with rage. Naruto has a strong sense of justice and the fact that someone took Hinata away from him would fuel his anger. He would vow to find and punish whoever was responsible. He might even become reckless in his pursuit of vengeance, endangering himself and those around him. His friends would have to step in to try and bring him back to his senses.
She would be devastated. Hinata has deep feelings for Naruto. His death would likely cause her to go into a state of shock and disbelief at first. She might blame herself for not being strong enough to protect him.
Hinata would probably be angry too. She might be angry at the circumstances that led to Naruto's death, whether it was an enemy or some other misfortune. She could direct her anger towards training harder, vowing to become stronger so that she can prevent such a tragedy from happening to others in the future.
Hinata might be shocked at first. She has always adored Naruto, and this sudden act of aggression from him would leave her stunned and unable to process it immediately.
He would feel a deep sense of guilt, thinking that if he had been stronger or more aware, he could have saved her. Naruto might also start to cherish the memories he had with Hinata more, and this could change his personality a bit, making him more somber at times. He would probably vow to protect the things that Hinata held dear, like her family and the village.
He would be filled with a deep sense of grief. Naruto has always cared about Hinata, and her death would be like a part of him has been ripped away. He might isolate himself for a while, blaming himself for not being able to protect her. But eventually, he would use his pain as motivation to get stronger and fight against whatever caused her death.
At first, Naruto would be stunned. But being Naruto, he'd quickly gather his wits. He'd probably joke around a bit to lighten the mood and try to get Hinata to open up. When he realizes she's serious, he'd start to analyze the situation. He'd think about what could have made Hinata do this and try to find a solution that would not only free him but also help Hinata if she's in some sort of trouble. He might also think about how this would affect their friends and the village as a whole.
Hinata would be overjoyed. She has loved Naruto for a long time, so his proposal would be like a dream come true for her. She might start crying tears of happiness and immediately say yes without hesitation.
Naruto, being the energetic and battle - loving guy he is, would likely be on the edge of his seat, eager to see who comes out on top in each death battle. He might be making bets with himself about the outcomes. Hinata, with her gentle nature, would be a bit more subdued. However, she would also be intrigued. She might be more concerned about the safety of the fighters, even though it's just a fanfiction battle. And when a fight gets really intense, she might clutch Naruto's arm without realizing it, while Naruto is too engrossed in the action to notice at first.
Hinata would likely be conflicted. On one hand, she still has feelings for Naruto, but on the other hand, she can't tolerate his arrogance. So, she would start by observing him from a distance. As she notices how his arrogance is affecting others, she would gather the courage to approach him. She would tell him how his actions are not in line with the values he once held dear, like friendship and protecting the weak. And she would hope that her words would make him reflect on his behavior.