Jiraiya's reaction might be complex. He might be in awe of Naruto's new power but also sad about his departure. He could see it as a natural progression in Naruto's journey, yet still feel a pang of loneliness. He might start to reflect on his own life and training methods, wondering if he could have done something differently to keep Naruto with him. At the same time, he would be eager to see what Naruto will achieve in the world with his newfound abilities.
Naruto would be determined to get to the bottom of it. He would not just accept the betrayal at face value. He would use his skills and the help of his friends to investigate what made Jiraiya turn against him. Naruto's ninja way is about never giving up, and this situation would be no different. He would fight against this betrayal, both physically if Jiraiya became an enemy in combat, and emotionally, trying to understand the reasons behind it all while still holding on to the hope that Jiraiya could be redeemed.
He would probably also feel angry. After all, he has sacrificed so much for the village and for them. He might start to question his own beliefs and values. But Naruto being Naruto, he would not give up easily. He would try to prove his innocence and show them that they are wrong. He could go on a journey to find evidence or seek help from other allies who still believe in him, like Shikamaru or Kakashi.
Naruto might be initially very disappointed. He has always looked up to Jiraiya and expected to be trained by him. So, when faced with the refusal, he could be heartbroken and feel like he has lost a great opportunity.
The Naruto fandom would likely be very divided. Some fans would be extremely outraged as it goes against the core values and character of Naruto. They might condemn such a fanfiction.
Sakura would be heartbroken. She had grown up with Naruto and always saw his potential. His death would make her question a lot of things. She might become more withdrawn and serious, vowing to become stronger in Naruto's memory. Kakashi would be deeply saddened as well. He had trained Naruto and seen his growth. He would likely feel responsible in some way too, as he was one of Naruto's teachers. As for Hinata, she would be shattered. She had deep feelings for Naruto, and his death would be almost unbearable for her. She might become more determined to protect the village in Naruto's name.
At first, Jiraiya would be in shock. He couldn't believe that Naruto has become so dark and left him. Then, he would be determined to bring Naruto back. He would follow Naruto's trail, trying to understand his motives. Along the way, he might face some challenges from Dark Naruto's new allies or the places he visits. But Jiraiya would not give up easily, as he truly cares about Naruto and wants to save him from the darkness.
Jiraiya might comfort Naruto by sharing his own experiences. He could tell Naruto about the times he faced difficulties and how he overcame them. For example, he might talk about his training with the toads and how he grew stronger through perseverance.
One possible reason could be that Naruto feels he has learned all he can from Jiraiya at that point. Maybe he wants to explore his own path independently and test his skills without Jiraiya's constant guidance.
Perhaps in this fanfiction, Jiraiya discovers that Naruto is going to be the cause of a major catastrophe in the future. He might have seen a prophecy or some sort of sign that convinces him Naruto has to die to prevent this. It could also be that Jiraiya has been misled by false information about Naruto, which makes him think that Naruto has become an evil force that needs to be eliminated.