In some cases, Naruto could be deeply hurt and withdraw from Hinata. He might question their relationship and whether he misjudged her all along. If the hurt is physical, he may also be more cautious around her in future interactions. For example, if she accidentally used a strong jutsu on him during training, he might be hesitant to train with her again without proper precautions.
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 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.
At first, Naruto would be completely shocked. He has always seen Hinata as a loyal friend and someone who has deep feelings for him. So the idea of her betrayal would be hard to fathom. He might try to deny it at first.
Naruto would probably be heartbroken. He loves Hinata deeply, so finding out she might have cheated would shatter him.
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.
Naruto might be initially heartbroken. He is a character who values loyalty and love deeply. So, when he discovers Hinata's cheating, he could be in a state of shock and disbelief.
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.
However, Naruto is also a very loyal and understanding person deep down. So after the initial shock and anger, he would probably start to investigate the situation more. He might talk to their friends like Shikamaru or Sakura to see if they know anything. He would want to get to the bottom of it rather than just making a hasty judgment.
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.