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.
The following are some novel recommendations about the reborn Zerg cubs, suitable for the female protagonist to read: " A Cute Child Descends from the Sky "-" The Cry of the Falling Leaf Worms ": This novel tells the story of a three-year-old cute child who brought the system to influence the villain. It was a warm and healing love story, very suitable for daily readers. Other recommended novels included The Law of the Insect Swarm, The Wonderful Life of a Male Insect, Ant Thief, Inexplicably Became the Prince of the Insect Race, I Became an Insect Enthrall After Rebirth, I Raise a Wife in the Insect Race, Gu Wei of the Reborn Insect Race, and so on. These novels covered different plots and styles, and readers could choose to read them according to their preferences.
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.