Encounter with Zodd
Casca resents Guts for making Griffith act impulsively and getting himself in danger for his sake. She is holding him responsible for the harm that was done to Griffith (despite of Griffith risking his life by his own choice).
When Guts encounters Zodd next castle siege, Griffith and Casca came to his aid as he was about to be torn apart.
Griffith then endangers himself trying to save one of his strongest men and tells Casca to withdraw the troops.
Look at Casca’s expression here when she notices WHY Griffith is risking his life. Is she sad that Griffith wouldn’t risk (or has not yet risked) his life for her in the same way he does to Guts…?
At some point Griffith reaches Guts and attempts to carry him out. They both assault Zodd, hurting him and cutting off one of his arms trying to get away, and then Griffith is being blown to the side by him.
Casca is in shock.
After Zodd saw Griffith’s Crimson Behelit, he punches a hole into the ceiling and gives Guts his infamous warning before taking off and exiting the scene. Then we see Casca running towards injured Griffith. Note how she is pushing Guts away from Griffith and giving him the blame for his injury.
She is holding him responsible and resents him for it deeply. We will see this later on as well, when Guts wants to visit an injured Griffith.
Note how she is looking away, avoiding eye contact to him. The tension!
Casca is stopping Guts from getting close to Griffith, as she has done before when he got injured during Zodd’s encounter. In this scene, I am also getting the impression she does not only act out of envy but also because she knows and respects rules of conduct, and Guts simply does not. She is reminding him of it and meets him with the same amount of stubbornness. Then wonders why Griffith would bother with a brute like him: “Why do you always have to be this way?”
Gutsca in the beginning wasn’t all flowers and roses at all.
Also note how Guts leaves here after that punch, stating he’d come back later, probably because he does not feel welcomed and does not want to upset Casca more than he already did (which makes sense considering she is crying here).
Looking at this, Casca has said and done a lot of hurtful things to Guts in the past.
Some of those things must be still ringing in his head even now, and it may be the main reason why Guts’ current companions all reached Elfhelm without a single causality, at the expense of his own well-being and humanity.
However at the same time let’s not forget why she did it, namely one part out of envy and the other part because of his stubbornness and utter disregard for rules and societal norms.