This is probably a hard pill for people to swallow, but Guts was never going to defeat Griffith in the end. Such a victory would be way too fanservice-y and dumb, especially with how much more cataclysmicly more powerful than Guts the Godhand are.
Even if Guts would’ve pulled off some Fairy Tail level bullshit about beating God Griffith in a fight, Guts himself would still feel pain in the end, killing or beating Griffith would not change that.
The only way Guts could’ve truly stuck it to Griffith, is by walking away and healing from him. Griffith is a narcissist who fed off of the anger/emotions that Guts felt toward him, only a complete reaction of indifference could’ve made Griffith truly suffer, and that would only come if Guts would’ve found the strength to walk away from Griffith in the end.
I think Miura foreshadowed this right at the beginning of the boat arc, where a very wounded Guts actually laid eyes on reincarnated Griffith, but simply closed his eyes and went to sleep as Griffith set out to face Shiva Ganishka. Guts and Casca were both beginning to heal from the trauma they suffered, going back to fight Griffith in the end, even if they win, would still leave them as nothing more than slaves to the pain that Griffith caused them, which would not complete Guts’ arc of wanting to find his own dream and his own reasons for living.
I never wanted to see Guts kill Griffith, I wanted to see Guts heal, which for me always meant not killing Griffith in the end.