One possible reason could be self - sacrifice. Maybe she was in a situation where stabbing herself was the only way to prevent some greater evil from happening. For example, if she was captured by the enemy and they were about to force information out of her that could endanger her comrades or the village.
It might also be a result of a broken heart or extreme emotional distress. If she had lost someone very dear to her, like Nagato, and she felt that she had no purpose left. In her despair, she could have resorted to self - harm as a way to end her emotional pain.
Perhaps she was under some kind of mind - control or a powerful genjutsu. The enemy might have manipulated her into stabbing herself as a way to weaken the Akatsuki or to send a message to her allies. In this case, she would not be in full control of her actions.
Well, perhaps in that fanfiction, there was a prophecy that was misinterpreted. Harry thought Snape was the key to a great evil about to befall the wizarding world. Also, it could be that someone Harry deeply cared about was seemingly killed by Snape's actions (even if it was a false perception), and his anger led him to this extreme.
Her friends or former comrades from Amegakure might be grief - stricken. They would remember Konan as a strong and important figure in their village. They might start to question the events that led to this situation and potentially start a search for the truth behind it.
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.
Perhaps she's under the influence of a powerful genjutsu. Genjutsu can distort one's perception and make them do things they wouldn't normally do. In this case, Kushina might be seeing Naruto as an enemy in the genjutsu and thus hurt him. Or it could be a test in a very extreme way. She might think that by putting Naruto through pain, she can make him stronger, although this is a very cruel way of thinking.
It might be a misunderstanding. Suppose there's a false rumor or misinformation that Naruto is a threat during the test. Sakura, being over - zealous in protecting the village or the test's integrity, decides to take him out. For example, if she believes Naruto has been secretly compromised by an enemy and she has no other way to deal with it during the test situation.
One possible reason could be a misunderstanding. Maybe Naruto was misidentified as a threat in a tense situation. For example, if there was some sort of dark jutsu at play that clouded Kurenai's judgment and made Naruto seem like an enemy. Another reason might be that Kurenai was under some sort of mind - control. There are many villains in the Naruto universe who are capable of such things. If she was being controlled, she might unknowingly attack Naruto.
Perhaps in the fanfiction, there is a plot twist where Sakura has been brainwashed by an antagonist. The antagonist could be using her to get to Naruto, knowing their close relationship. So, she would then be made to attack Naruto without her own will. Or, there could be a prophecy or some sort of pre - determined fate in the fanfiction's plot that causes Sakura to hurt Naruto, even if she doesn't really want to.
It might be for the greater good. If Obito's actions in the fanfic are leading to destruction on a large scale, Konan, being a protector of sorts, decides to take him out to save the world or at least her part of it.
Another reason could be related to a prophecy. If there was a prophecy in the fanfiction that Naruto would bring great destruction in the future, and Kushina, being a self - sacrificing mother, might think that killing him is the only way to prevent that destruction, even though it goes against her natural love for her son.
One possible reason could be that Naruto has some sort of new power that seems menacing or uncontrollable to Sakura at that moment. Maybe it's a power he got unexpectedly and it has some sort of dark or wild aspect to it that makes her blurt out 'demon'.