Guts the rapist is the villain of the story. He must lose in order for the story to end well.
Guts forcefully penetrated a mentally ill Casca as can be seen in this image. That's the behavior of a rapist who penetrates the woman disregarding her will. No, blaming some "Beast of darkness" who came from Guts himself does not make him any less guilty.
Casca is just a way for Guts to keep hating Griffith, which Kentaro Miura confirmed:"[...]That's why I spared Casca. [...] I feared the reason for revenge would become something of the past[...], but it's because Casca's by his side that he can never forget the Eclipse.".
Griffith haters love their comfort and being told what to do and think, which is why they act like a mindless herd. The same kind of herd that blindly follows Griffith in the manga. So Griffith haters are hypocrites.
Casca does not share Gut's desire(vengeance). She loves Griffith, the man who saved her life and who loves her as much as a child loves her mother. Casca will be most happy living in the castle as Charlotte's mother in law.
LONG LIVE GRIFFITH
MAY GRIFFITH LIVE HAPPILY ALONGSIDE CASCA
MAY GUTS FORSAKE HIS REVENGE LEST HE ENDS UP LIKE GANISHKA
