This novel was called "Blade Breaks the Demonic Sky". The main character said,"If I break the Demonic Sky to prove my true self, I will smile at the sky with my blade, and slaughter all the pigs and dogs in the world to lower my head." I recommend this fantasy novel to you. The story is fascinating and the details are well handled. I believe you will like it. I hope you like this fairy's recommendation. Muah ~😗
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.