For Sasuke, it could be a huge shock. Since they are rivals and friends, seeing Naruto do something so extreme would make Sasuke question his own role and his relationship with Naruto. He might become more determined to find out what led Naruto to this and seek revenge on those responsible. Sakura would be devastated. She has always cared for Naruto and seeing him harm himself in such a way would make her feel powerless and guilty. She might try to use her medical skills to fix the situation, even if it seems impossible.
Perhaps it's related to some sort of new power - up or transformation in the fanfiction. Sometimes in fan - made stories, the author likes to create unique and extreme situations for character development. Naruto might think that by removing his eye, he can unlock a hidden power within him. It could also be due to a misunderstanding. He might be tricked into believing that his eye is the source of some great evil that is about to befall the Hidden Leaf Village and so decides to rip it out to save everyone.
In this fanfiction, perhaps Naruto has seen a side of Sakura that goes against his values. For example, if she starts to use underhanded tactics to get what she wants. Naruto, being the person he is, doesn't tolerate such behavior and he confronts her, really tearing into her actions and motives. It could also be related to her treatment of him. Maybe she has been dismissive of Naruto's feelings for her and he finally snaps and gives her a piece of his mind.
Well, it could be due to a powerful enemy attack during a fight in Wave. Maybe an enemy with a sharp weapon or some dangerous jutsu managed to strike Naruto's eye.
Another reason could be related to his personal growth. He might be trying to master the power of the eye in private. Naruto has always been one to face challenges alone at first. By hiding his eye, he can focus on understanding and using its power without distractions or the pressure of others' expectations.
One consequence could be that his friends might start to suspect him. Since Naruto is usually an open book, hiding something as significant as his eye could make them wonder what he's up to.
In this fanfiction, perhaps there's a big misunderstanding. Bucky has been led to believe that the arc reactor is some sort of key to a greater evil. So, without fully understanding its importance to Tony, he rips it out. This could then lead to a chain of events where Bucky has to face the consequences and try to make amends while Tony is fighting for his life.
It could be Bucky himself if he's under mind control. He'd be the one directly causing harm to Tony by ripping out the arc reactor, so in that sense, he's the antagonist, even if it's not really his fault.
One possible consequence could be a change in his fighting style. Without his eye, he might have to rely more on his other senses like hearing and smell, similar to how blind ninja in the Naruto universe adapt. For example, he could focus more on using his chakra to sense the movements of his opponents.
It could be Sakura herself in a sense. Since Naruto is ripping into her, she is the one causing the conflict. Her actions or words might be what Naruto is reacting against, so she could be seen as the antagonist in this scenario.