It might be a jealous rival. If Sakura is popular or has something that others envy, like a special talent or a relationship, a rival could have stabbed her out of envy.
Perhaps Sakura's injury makes her realize she has some hidden strength within her. She could start to have visions or gain new abilities while she's recovering from the stabbing. This could then lead to a whole new arc in the fanfiction where she has to learn to control these new powers.
I'm not sure specifically as there could be many different interpretations in such a fanfiction. It might involve a character named Rito facing a stabbing incident, perhaps in a battle, a crime scene, or a situation of betrayal.
It could be a rogue angel. There are always some angels in the Supernatural universe who oppose Castiel's ideas or actions, and they might see stabbing him as a way to get rid of him or send a message.
In such fanfiction, characters might show shock. For example, Dean could be horrified when he sees Castiel stabbed, his eyes wide open and his face pale.
Well, in 'naruto gets stabbed fanfiction', it could be that Naruto is exploring a new and dangerous area where he encounters a powerful and hostile ninja. In the heat of the fight, this ninja manages to land a stabbing blow. It might also be that Naruto is weakened due to some prior injury or chakra depletion, making him more vulnerable to being stabbed by a regular enemy. And, there could be a situation where Naruto is under the influence of some sort of genjutsu that impairs his ability to defend himself and thus gets stabbed.
Perhaps he gets stabbed as a part of a plot to take over his company. His rivals could be behind it, seeing his success as a threat. In the fanfiction, this could lead to a great revenge story where Kaiba has to regain his power and seek justice.
It's not okay. Abuse is a serious and negative concept. Fanfiction is a form of creative writing, but when it involves abuse, it can have a negative impact on the perception of the characters and also be triggering for some people who may have experienced abuse in real life.
I'm not aware of any such fanfiction. This concept might be quite unorthodox and not in line with the general themes in many mainstream fanfiction communities. Also, it could potentially involve inappropriate or non - canon interpretations of the characters.