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 main character in Journey to the West, Sun Wukong, was rebellious, brave, resourceful, impatient but with a sense of justice, emotional, loyal, daring, and abhorrent. He was born in the Water Curtain Cave of the Flowerfruit Mountain. He was smart and talented since he was young. He had taken Patriarch Subhuti as his master and had learned many powerful spells. He had the ability to transform into the Seventy-Two Transformations, and he also had the Golden Cudgel and other magic treasures. Sun Wukong was unruly and unyielding. He was unwilling to be bound by any restrictions. Therefore, he dared to cause trouble with the Sea Dragon King, the Infernal Palace, the Heavenly Palace, and other forces. He even made a bet with Buddha. He was full of vitality and courage to face challenges, showing the characteristics of a hero. However, he also had the weakness of being competitive, arrogant, and impatient.