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 haircut could be significant in terms of character development. For instance, if a character was very conservative before and then gets a wild haircut, it shows their evolution. It can also be a way to show the character's mood or intention. If someone is getting a haircut before an important event, it might signify their determination to make a good impression.
In a sexy haircut story, novelty can make a haircut sexy. If it's something different from the norm, like a shaved side with long hair on the other side. It catches people's eyes and gives an edgy, sexy vibe. Another factor is how it complements your overall style. If you have a rocker - style wardrobe and you get a spiky, short haircut, it goes well together and looks sexy. Also, a haircut that shows off your neck or collarbone in an elegant way can be considered sexy.
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.