Casca takes the deal. There'd be no saving her from that. Whether he can bring himself to kill her or not, Guts would be demoralized, and his consistent savior to relinquish the Beast's hold on him would be gone.
I do believe that Casca is the character who is most likely to use Guts' Beherit and this is why.
The Beherit that Guts is carrying is basically a
https://en.wikipedia.org/wiki/Chekhov%27s_gunThere's no reason for that Beherit to be there unless it will be used by someone in Guts' party. I'm just saying that Casca is more qualified to use it than anyone else in Guts' party.
What's the most important lesson from the Eclipse? That sometimes people
can't go back to how they used to be.
All the warning signs are there that something bad might happen after Casca's mind is "fixed":
1. Nobody asked Casca if she wants to be cured. That's probably the most important thing about all this. Guts, Farnese, and Schierke may think that they are helping Casca, but does Casca see it that way?
2. Guts didn't tell his new comrades anything about what really happened during the Eclipse or about Casca's miscarriage or about their child. Schierke might have some ideas about what happened (although she is obviously wrong about the moonchild being the Elf Queen's avatar) but Farnese is totally clueless. Are they really qualified to fix Casca's mind? I'm not so sure. I believe that only Guts can truly help Casca, but the problem is that Casca now hates and fears him.
3. What exactly is being hinted at in Episode 202 when Guts asks Flora to teach him how to use the Beherit and we see Casca suddenly run into the room. Why is she there? Guts isn't going to sacrifice Casca (or anyone else) so what's Miura trying to tell us in that scene? It's not like Casca suddenly wanted to be with Guts, she already hates and fears him at that point in the story.
4. Casca isn't 100% mentally disabled. She was able to safely jump down the cliff in the Conviction story arc, she was able to pick up a sword and kill those bandits who tried to rape her in the Falcon Of The Millennium story arc, and she is clearly able to recognize her child. We even see her feeding the moonchild in Episode 238. What happens if Casca really didn't want her mind to be "fixed"? Will she get angry? Will she become irrational and hostile to the party? Even Golden Age Casca was emotionally disturbed and suicidal at one point. She just isn't as strong as Guts physically and mentally.
We have seen many times in Berserk that people can't escape their fate no matter what they do. I understand why many fans might not like such a tragic outcome. But even if most fans don't like it, that doesn't make it illogical.
I am certainly not suggesting that Casca will suddenly turn into an evil, vengeful bitch and sacrifice Farnese just to get a power up. Of course not, that would be stupid. Casca will have to hit rock bottom and be at a point of no return before she uses the Beherit. If it happens, it happens. I expect that Miura will not disappoint us. The Skull Knight did warn Guts that Casca may not want what he wants. Miura has never mislead the audience before. He basically told us back then that there won't be any happy ending for Guts & Casca.
And we have seen that the Godhand can be very persuasive. They don't exactly lie but they do twist the truth to convince the person who summoned them to accept their offer. Casca does have something that her new friends can't help her with, her child.
Again, it's not so implausible that the Godhand might show Casca visions of her child and convince her that the only way she can "save" her child is by ascending beyond normal human limits. It doesn't help than one of the Godhand is a woman. Slan might convince Casca that:
1. she can't rely on others
2. the men who she loved have only hurt her
3. a good mother will sacrifice anything for her child
Golden Age Casca would never accept anything the Godhand have to offer but that version of Casca doesn't exist anymore. She is now a rape victim and a mother who lost her child. Her mind regressed to an infantile state for a good reason.
Guts is the only person there who truly loves Casca and knows everything about her. That's undeniable. I believe that Guts is far more qualified to help heal Casca's mind than some kid who barely knows her or some friendly noblewoman who knows almost nothing about what Casca has been through. So it looks very strange that the Elf Queen (who supposedly knows everything) already refused to use the power of love to cure Casca in this magical therapy session. It almost looks like the therapy session is already set up to fail badly, not because the Elf Queen has any malicious intentions but because it has to fail to advance the story.
And I don't buy the explanation that the all-powerful Elf Queen (who knows everything) somehow can't use her magic to calm Casca down so that Guts can participate in the therapy.
And just to remind everyone how unqualified Schierke is to help Casca during the therapy, see Episode 252. She couldn't even help Casca bathe. Casca doesn't listen to her and doesn't obey her at all. Farnese is more qualified than Schierke but see Episode 288. Farnese doesn't understand why Casca hates and fears Guts. What is Miura telling us here? That Farnese has good intentions but she doesn't really
know Casca at all.
Of course we know that the Godhand are evil bastards who couldn't care less about actually helping Casca but does Casca understand this? What she knows is that the Godhand took Griffith's broken, incapacitated body and gave him a new life.
Casca does remember how the apostles raped her but this is unfortunately yet another reason why she might accept the Godhand's offer now. Casca might feel that she must become stronger at any cost so that nobody can hurt her again. She isn't stupid. Even if she is given a magical weapon I don't see her getting very far. Hell, Serpico and Isidro have magical weapons but they didn't kill a single apostle during the battle in the Spirit Tree Forest, and there's no spare set of berserker armor for Casca to use.
What is Casca's dream now? She wanted to be useful to Griffith but that dream is over. I think that now Casca just wants to be with her child but she also wants to be strong enough so that men and demons can't hurt her ever again. Looking at this from her point of view it actually makes a lot of sense for her to ascend beyond human limits.
And yes I definitely understand why many fans don't like this idea at all. They want Golden Age Casca back. They want Guts and Casca to become lovers again. They want Guts and Casca to work together to defeat Griffith and then settle down and live happily ever after. But this is Berserk that we are talking about. Guts not dying in the end would be a happy ending.
Speaking of what may happen in Elfhelm, we know that Griffith considers all magic users to be a threat. But his army
can't launch a surprise attack on Elfhelm because all the wizards and witches can sense od. They would be able to sense the evil od of Griffith's soldiers from miles away. The archmages and the Elf Queen are certainly smart enough to expect and prepare for an invasion from the outside. So what's the best way to destroy Elfhelm? Have someone (Casca) use the Beherit to summon the Godhand
in Elfhelm.
We haven't yet seen members of the Godhand fight a real battle. We don't know what they are truly capable of but it's obvious that they are much more powerful than the apostles. If anyone can destroy Elfhelm, it's them.
Nothing is a coincidence in Berserk. It's not a coincidence that Guts obtained the Count's Beherit. It's not a coincidence that Zodd destroyed the elf cave thus forcing Guts and Casca to travel to Elfhelm in the first place. And it's not a coincidence that Farnese in now Casca's best friend and easily qualifies as a sacrifice.
Guts was warned that it's dangerous to own and carry a Beherit but he didn't listen, something bad has to happen sooner or later.