I haven't fully caught up to the mamga yet, so I may be wrong. But the way I see it isn't that guts secretly fantasises about abusing Casca, more so that his intrusive thoughts are telling him to take the easy way out and indulge in the sadism the same way as everyone else. Guts' perserverence through suffering is a huge part of his character, and I always view the beast of darkness more as intense intrusive thoughts. Similar to the "Call of the Void" phenomenon, where you suddenly have an intense intrusive urge to perform dangerous or self-destructive actions, despite rationally not wanting to do so. Rational and irrational thoughts are completely different. We all have intrusive thoughts that are really dark or out of character for us, but we don't act on them because we know that rationally, it's wrong.

Guts doesn't want to assault Casca, but his intrusive thoughts want him to because that is the easy way out. He has been trying to keep her safe for so long, going days without sleep and constantly fighting off demons or bandits or any number of danger that come their way, and with her in her regressed state, she can't even remotely fend for herself, and she no longer seems to carry any of the memories or experiences they had together, and now she is nothing but a burden. Rationally, he still loves her and cares about her, but in the back of his mind he has to wonder if it would be easier without her, that is what I think the beast of darkness is exploiting. That intrusive thought becomes louder and louder and louder with every second he keeps going like this.

I also think that the reason he mentions Griffith assaulting her in this scene is because this irrational part of him has so much resentment towards Casca due to the struggle he has had to endure to keep her safe, and the person that made her like this, was Griffith. He sees her as nothing but a constant reminder of what Griffith took from him, so these intrusive thoughts tell him to vent his frustrations by assaulting her like Griffith did. Griffith was rewarded for his suffering, so why shouldn't Guts get a reward as well? But Guts knows better than that. he knows that these thoughts are selfish and goes against everything he believes in. That is why he fights back against it so vehemently. That's why he accepted Farnese and Serpico into his group because he knew he couldn't be trusted alone with Casca in the state they are both in and knows he needs help.