Casca killing Griffith...?

[Pez]

Nah, that's not how he views it himself. His state of mind in that regard is perfectly embodied by the Beast of Darkness: he'd rather die trying than give up. He's not looking at it in a calm and reasonable manner. For example, in Vritannis Serpico had to intervene while a half-dead Guts was standing up to Zodd and a bunch of apostles, ready to start a quarrel he couldn't have possibly won. Casca was the reason he stepped down, as always.

If she weren't there Guts would most likely be dead already, having tried to get to Griffith amidst a sea of apostles.

That is a very good point. But Guts himself is not a very calm and reasonable person (although he does have some such moments), so I feel as though I cannot really expect him to make calm and reasonable decisions. He has changed a lot, however. I just cannot fathom how things will end if he truly gives up any thoughts of revenge and tries to live happily with Casca...

 

[Aazealh]

But Guts himself is not a very calm and reasonable person (although he does have some such moments), so I feel as though I cannot really expect him to make calm and reasonable decisions.

Depends, sometimes he does actually. More often than people think. However when it comes to the God Hand or even apostles he just can't help himself.

I just cannot fathom how things will end if he truly gives up any thoughts of revenge and tries to live happily with Casca...

Well obviously the story can't end on that note. He will eventually have to face Griffith again, but for what reason, that is Miura's secret for now. We'll just have to wait and see what he's concocted us.