Frisk has been watching, with their hands clasped over their mouth like a horribly-timed parody of Toriel's expression of shock and regret. They even cried out when their Mirror seized the mirrored version of Chara like that, terrified that they might kill them, forgetting that the conduit is one-way, and flipped around. They're already so fragile and breakable, and this...this isn't fair for either of them!

Their mirrored self really is cruel. It's not like they're Chara, either, easy to shuffle the blame. They're Frisk, the way Frisk was on those darker, colder timelines where they cut down everything in their path without a shred of remorse.

Frisk pulls themselves together, sniffling pathetically, and retrieve their marker to scribble on the mirror's surface:

* I'm sorry. I wish I could've stopped them.
The asterisk is unnecessary. They never need it, not being who and what they are, but...maybe they need some distance here. Maybe they both do.