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.
Here are a few recommendations for novels that turned black after being betrayed:
1. " The male protagonist is always blackened ": In this sci-fi space-time travel novel, the male protagonist is often bullied and isolated, and eventually blackened.
2. " Immortal Evil Dragon of the Heavens ": This heavenly novel tells the story of the protagonist's dark side after his death in a conspiracy.
3. " Man in the Heavens: Becoming Stronger When You're Darkened ": This novel described the changes of the characters after they were betrayed, including the fake darkening and the real darkening.
4. The protagonist of this novel went through a cruel betrayal and torture before turning completely dark.
I hope you like the novels recommended above.