If that's all you got out of it then I think you missed a whole hell of a lot as to what that event meant in terms of the character relationships that had been built up between Guts, Casca and Griffith and what it also meant for the story going down the line. Rape is terrible and that was the whole point. Berserk uses things like Rape, outside of the trolls, to signify how a terrible event or moment is only getting worse. Its a series with wall to wall graphic depictions of gore, violence and human suffering. What Femto does to Casca is basically to clue the desensitized by everything else that,"Oh shit, things are getting REALLY bad" and as gross as it is to say, is a big part of his character building. He has thrown away his humanity, had it burned out of his soul and what best way to prove it while also getting back at the guy who almost ruined your dream.
Edit: I will also say the series doesn't try to be tasteful with it either and that is often times on purpose especially when relating to someone like Wyald.
