Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.
Perhaps in this fanfic, Griffith has turned into something truly evil and a threat to the world Guts cares about. Guts, being the warrior he is, might feel it's his duty to put an end to Griffith to save others. Also, Guts could be driven by his own sense of justice. If Griffith has wronged innocent people, Guts may decide that killing him is the only way to set things right.
It depends on the specific fanfic. Some fanfics might have Guts killing Griffith as a major plot point, while others could take a different direction and not have that happen at all.
The characters included the male protagonist, Wu Qingliu, the ruler of the Forsaken City of the Gods, the survivor of Canglan, and the silent avenger.
"World Eclipse" by Yu Yao. It was a fantasy/mystery fantasy novel with elements of transmigration and leveling.
Author:
The void floated and sank for a few worlds, and the rest of his life entered Canglan without regret.
Wu Qingliu thought,'Am I the lone star of the Heavenly Fiend? Should the main character be this rough? This is a bad year. Is it easy for me to raise a daughter and a split soul? How many words did you say and you let me meet a black hand? If you don't send some strong support to fool me, I'll show you how useless I am. No matter what, you have to let me develop first and put on my godly equipment before I go solo.
The Lord has awakened ~ The story begins.
I hope you will like this book.