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.
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.
Yes, there are many Berserk fanfictions that focus on the complex non - romantic aspects of Guts and Griffith's relationship. For example, some may explore their friendship before the betrayal, their rivalry on the battlefield, or their mutual respect for each other's skills. These fanfictions can provide a deeper understanding of their characters and the overall story of Berserk.
I'm not sure specifically as there could be many interpretations of such a fanfic. It might involve a scenario where Guts, a character, kills Griffith in a fictional story created by fans. Maybe it's about Guts seeking revenge on Griffith for some past wrongs in the context of the original story's lore.
In some fanfictions, their relationship can be seen as a source of inspiration. Griffith's grand vision might inspire Guts to strive for something greater than just survival. At the same time, Guts' unwavering determination can be an inspiration for Griffith to keep pushing forward in his own goals, all without any romantic or sexual elements.
Friendship is a big theme. Despite their differences, they had a strong bond at one point, and fanfics may develop this aspect more, showing how their friendship could have evolved in different scenarios.
Yes, there are some. You can start by looking on fanfiction websites. Some might be based on different interpretations of their relationship in the original work.
Well, 'A New Path' is also a great 'Griffith x Guts' fanfic. It takes a different approach by imagining a world where certain events in the original story didn't occur. This changes the dynamic between Griffith and Guts completely, and it's really interesting to see how their relationship develops in this alternate universe.