Griffith desires a kingdom above all. That's what pretty much all of the Golden Age Arc revolved around. During this arc, you could even say that Griffith IS the PROtagonist of the story. It was all about the pursuit of his dream of being the one on top of a castle.
Now there is much more back story & lore behind those final moments of the arc in the berserk universe, but the basic gist of it is... Given Griffith's predicament where he could no longer pursue his dream in his broken state, he ultimately made the choice to sacrifice his former comrades so that he may be reborn and therefore be able to continue that pursuit. But the conflict of reasoning behind that choice and why he ultimately made the one that he did is what encapsulates the best of the Golden Age Arc. Some further insight into that below.
Why did he end up making that choice? Well, beyond the obvious reason of his mutilated, and therefore useless, body... And why did it cause him to turn against, and target Guts in particular, after he became Femto? Would you have made the same choice? These questions make up the best part of berserk in my opinion.
So, would you have made the same choice? It's commendable if you wouldn't, but understandable if you would. The story shows how Griffith ultimately came to his own personal decision, and all I can say is it's got less to do with his body and more so to do with what he's already sacrificed, even before the Band of the Hawks, as well as just being the type of person he is.
Last, but not least, Griffith's actions against Guts as Femto is also a lot deeper than many people realize. As he made his choice, Griffith admits that Guts is the one who, thru comraderie and perhaps even friendship, made him forget about the pursuit of his dream even for but a short time. This, especially for a character like Griffith, is almost like an assault to his very nature. Therefore, it's no wonder that Griffith, reborn as Femto, practically devoid of most, if not all of his humanity, targets Guts for this "transgression". And yes, the biggest part to this was raping Casca in front of him.
If you rewatch the show again, keep this post in mind.