I've probably said before how I think Claw would've been stronger had it left out the last two chapters and just ended with Natalie waking up in the hospital. I don't think that's an uncommon opinion on here—regardless of how one feels about the ending, it does seem to take a very jarring turn away from the pacing, focus, and to a certain extent characterization.
That said, if we needed to have Mia on trial, then execution aside, having Gio be the prosecution was a pretty good move. There's some ways their relationship progresses, like the implication that we should read the Hursts as grooming Gio towards violence in the same manipulative way Davie groomed Addi, that get a bit lost here in the final instance. But the big idea that Gio eventually gets drawn to—her role as the "little shadow," the person who finds themselves echoing whoever their around and picking up their ways of thinking reflexively—allows her to be both a character that Mia has helped immensly and gotten close to, as well as a character who when removed from Mia quickly picks up the "common morality" that would brand her an uncomplicated monster.
I maintain that the best reading of the last chapter is not that Mia is finally getting a "fair trial," that wildbow is telling the audience that we should see Mia as objectively a horrible person. Instead, its an expression of Mia needing to be seen as non-monstrous by the world when her actions are laid bare, and the world refusing her. She's pushed away by Gio because she's a strange and cold person who's hard for normal people to really see as a person, and because its easier to go about your life if you just shake your head, call her a monster, and removing her from your life. Just repeat back what everyone else would say about her. Even if you have the strongest reasons out of anyone to deny that she has an uncomplicatedly evil character, its so much simpler to look past all that. Blithely repeat the standard line on parental rights, even though you've been hurt by such rights worse than anyone. Repeat a barely-thought-out comparison between the Hursts and indigenous residential schools, even though it's insulting to all parties if considered for more than five minutes—even so, others are saying it, and if you believe it the break will be cleaner.
Mia got a judgement from someone who she did an incredible amount to help, and was found wanting. But ultimately, she was just getting the same judgement of the society who shut her out after the fall. The little shadow becoming a shadow of Mia's mother, looking at her like she'd killed her daughter and stolen her skin.
