Let me explain why I'm fascinated by Saikoroshi. And how this episode contains a line that haunts me.
Saikoroshi is the story of Rika who is sent in an alternate reality. In this world, she doesn't have any friend. She's bullied at school. She lives with her parents, and they're relationship is tense.
Because it's a story with two realities, it can be taken on two perspectives. The first, straightforward, is the perspective of Rika. She's rejecting this world. It's not hers. It's not her friends. She took the place of "Rika Furude" of this world and killed her.
But the perspective from people of this world is compelling. As she acknowledge, nothing looks supernatural to them. It looks like she's just an isolated girl.
And OMG, when Rika talks about Hanyū to Dr Yamamoto, he asks to see her. Rika shows the crystal ball that she uses to communicate with her... and Hanyū doesn't talk. Conveniently, the ball is not charged enough. From his perspective, it's exactly what a girl with an imaginary friend would say.
As we discussed with @anawkwardlady, this episode can be taken as a metaphor of dissociation, with depersonalization. There is a link to make with maladaptive daydreaming, but I'll link her post.
Now, it's time to go to the line
It happens during the discussion between Satoshi, Mion, Reina and Rika/Bernkastel. I've included two screenshots for context, but look at what Reina says.
"Rika-chan, you think it doesn't have anything to do with you. ... Am I right?"
Rika uses dissociation to distance herself from the world and her responsibility. But the narration won't let her.
I think that I understand now: "Frederica Bernkastel" did not kill "Rika Furude". They share the same personality, the same flaws. (Important disclaimer: I'm relating what Ryukishi says, but I don't agree. No, I refuse to blame Rika for her bullying.) Whatever "Frederica" says, it's still about her.
There is no contradiction between the two perspectives I talk before, and they merge at this line. (Again, brilliant VN even if I don't agree with all of the moral of the story.)
To sum up, Ryukishi makes a story about alienation by literally sending Rika in a universe she doesn't belong to. It could weaken his point (like "of course none of what happens is about her"), but no, he develops the story in the most clever way possible. It will never cease to drive me crazy.