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.
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.
For example, in an anime where the protagonist's friend is killed by the enemy and then goes berserk, you can refer to the following example:
" The Blade of Ghost Destruction ": The story tells the story of a villain organization called Onimaki Tsuji Asamatsu, who turned the entire Japan into ruins in order to achieve his goal. The protagonist, Ashijiro, is a young man who fights against Onimaki Tsuji Asamatsu in order to save his family.
In the battle between Azhiro and his friends, Chihiro and Orochimaru, they were attacked by the enemy. Chihiro was unfortunately killed by the enemy in order to protect Azhiro.
When Tanjiro learned of Qian Xun's death, he began to lose his mind and fell into extreme grief. After Chihiro's death, Akashiro's body began to become abnormally powerful and he was given the title of "King of Ghost Hunters". He began his journey to save the world.
In the process, Tanjiro continued to fight against various enemies and continued to grow in the state of rampage. In the end, he successfully destroyed the ghost dance and saved the whole world.