Sakura might be initially shocked and then start laughing uncontrollably. She would probably tease Naruto about it for a while. Sasuke, on the other hand, would likely just give a slightly raised eyebrow and then act as if it's not a big deal, but deep down he might be a bit amused. Kakashi would probably make a dry joke about it.
Shikamaru would likely find the whole situation tiresome. He might think it's a bother and wonder why Wendy has to have a crush on Naruto. Sasuke would probably be indifferent at first. He has his own goals and isn't really interested in Naruto's love life. However, if Wendy's crush starts to interfere with Naruto's progress in some way, Sasuke might step in and tell her to back off. Naruto's friends like Choji and Kiba would probably tease Naruto about Wendy having a crush on him, which could make Naruto a bit embarrassed.
For Hinata, it would be a crushing blow. She loved Naruto so much. She might withdraw into herself at first, overcome with grief. But then, she could find the strength within herself to carry on Naruto's legacy, especially for their son, Boruto (if in a post - Naruto - has - a - family scenario in the fanfiction). Shikamaru, being Naruto's friend, would feel a deep sense of loss. He would use his intelligence to try to figure out what led to Naruto's death and if there was any way to prevent it in hindsight. And the other members of the Leaf Village would be in a state of shock and mourning. Naruto was their hero, and his death would make them question their safety and future without him.
The other Konoha ninjas would be quite shocked. The Hokage, whoever it is at that time, would see Naruto's change as a potential threat to the village's stability. They would likely keep a close eye on him and try to figure out what caused this transformation. Meanwhile, the younger generation of ninjas who looked up to Naruto would be confused and disappointed. They expected Naruto to be their role model of a kind - hearted and positive ninja, not someone acting like the feared Madara.
Sasuke would be deeply affected. He and Naruto had a complex relationship. He might feel a sense of regret for all the times he had pushed Naruto away. His reaction could range from anger at the situation to a new - found respect for Naruto's sacrifice.
Sakura would be completely devastated. She's been in love with Sasuke for so long and has a deep bond with Naruto too. She might go into a deep depression and lose her will to be a ninja for a while.
Sasuke would be extremely suspicious. He has always seen Naruto as his rival and a symbol of the village's hope. If Naruto starts acting like Pein, Sasuke would think it's some kind of trick at first. He might even be ready to fight Naruto to figure out what's really going on.
Sakura might be initially shocked but then she would be determined to help Naruto find out what's wrong. She could start researching in the hospital's library for any medical or historical records related to such a phenomenon.
Sasuke might be conflicted. He has his own complex relationship with Naruto. Part of him might understand Naruto's reasons for leaving, but he could also be angry that Naruto didn't confide in him. Sakura would be worried sick. She has always cared about Naruto, and his leaving would leave a big hole in the team. Kakashi, on the other hand, would be disappointed but also respect Naruto's decision if he thought it was for a noble cause.
Iruka - sensei would be very concerned. He's always been like a father figure to Naruto. He'd probably spend a lot of time with Naruto, sharing stories from Naruto's past when he was brave and encouraging him to find that courage again. Kakashi - sensei would be more analytical. He'd observe Naruto closely, trying to figure out if this is a mental block or something more sinister. And the other villagers might start to lose faith in Naruto as their hero, which would add more pressure on Naruto to overcome his fears.