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.
One key indicator is the level of detail. In facts, there are usually specific details. For instance, a father talking about his first car will mention the make, model, etc. For a student, if it's a fact about a project, they'll say what materials they used. Fiction often lacks such details or has over - the - top, unrealistic ones. Another is the purpose. Facts are to inform, while fiction is mainly for entertainment.
Well, for father's stories, facts might be based on his real-life experiences like his job or family history. For students, facts in their tales could be from school lessons or personal observations. To differentiate from fiction, look for consistency. If a story keeps changing details, it might be fiction. Also, check for sources. If it comes from a reliable book or personal memory, it's more likely a fact.
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.