Well, without more context, it's hard to say precisely. 'Percy' might refer to a character from a particular series, like Percy Jackson perhaps. And if it's a fanfiction about his mom killing a young character, it could be a very dark and alternate take on the story's universe. Maybe the writer was exploring some extreme plot ideas, or it could be part of a more complex story arc within the fanfiction community that deals with themes of violence and family in a different way than the original source material.
In the anime," Street of Souls," there was no clear explanation of who killed Tang Xiaoxiao. Although Tang Xiaoxiao was described as a girl with special abilities in the original novel, the anime did not directly show her special abilities, nor did it explain how she was killed. Therefore, the anime did not give a clear explanation for Tang Xiaoxiao's death.
One possible motive could be jealousy. Maybe Brett felt that his mother favored his sister over him. This feeling of being second - best might have festered inside him until it exploded into this tragic act.
I'm sorry, I haven't been able to find any examples of this specific 'percy mom killed young fanfiction'. It might be a very niche or even privately - shared piece of fanfiction.