Kakashi would be the most perceptive. He'd see how Naruto singing 'Monster' is not just a random act but a way for Naruto to connect with his own fears and insecurities. His friends from the Hidden Leaf Village would overall be proud of Naruto for trying something new and for using the song to share a part of himself that they may not have fully understood before.
It could be Orochimaru. He might see Naruto's 'monster - like' state as an opportunity to use him for his own experiments. Orochimaru has always been interested in power and Naruto's new, strange form could be a source of great power that he wants to harness for his evil plans.
Perhaps in the fanfiction, Naruto is under a genjutsu that makes him see himself as a monster. When he sings 'I Feel Like a Monster', it's a cry for help. His friends start to notice his strange behavior and try to break the genjutsu. Along the way, Naruto's singing might draw the attention of an enemy who wants to use his 'monster - like' state for their own evil purposes.
His friends' reactions would be quite diverse. Kakashi, for example, might be intrigued by Naruto's new behavior. He could see potential in Naruto's new - found Kisuke - like qualities. The younger generation of ninja, like Boruto if he were around in this scenario, would look up to this new version of Naruto even more. They'd be inspired by his new cool - headedness and intelligence, while also being a bit in awe of his sudden change in demeanor.
His friends' reactions would vary. For instance, Choji might be excited and say, 'Wow, Naruto! You look so cool like Minato - sensei!' Ino would probably comment on how much more handsome Naruto looks and how it gives him an air of authority. Kiba might tease Naruto a bit at first, but then admit that it's a great look. Hinata would likely blush and be shy, thinking to herself how much Naruto resembles his strong and noble father.
There could also be friends who are so loyal to Naruto that they would consider following his request if they truly believe it's for the greater good. However, they would be deeply pained by it. Kakashi, for instance, might understand Naruto's selfless nature but would also struggle with the moral and emotional implications of taking his life.
Sasuke would be furious. He already has a strong sense of justice and loyalty to Naruto. He'd likely immediately start looking for Naruto, using his Sharingan to track any possible leads. He might even go against the village rules if he thought the village was involved in Naruto's kidnapping.
Sakura might be really surprised at first. She could offer to help Naruto with his girl disguise, teaching him things like how to do his hair or choose the right clothes. She might also find it a bit funny but would be supportive in the end, seeing it as part of a mission. Sasuke, on the other hand, might be a bit more aloof. He would probably just raise an eyebrow at Naruto's new look but wouldn't make a big fuss about it as long as it was for a good reason.
His friends would be shocked at first. Sasuke might feel a sense of responsibility to find Naruto and bring him back. Sakura would probably feel guilty, especially if she was somehow involved in the situation that led to Naruto being cheated on.
There might also be a sense of self - doubt. Naruto may start to question his own judgment. He thought his friends were true to him, so the betrayal could make him wonder if he was wrong about them all along. He might also doubt his own worthiness as a friend and leader, which could have a long - term impact on his self - esteem.