Best way to explain it is that Mitori is just that unhinged for Dolly.
Also I feel like the theme is that every esper even involved in the dark side didn't do anything wrong since it was the adults who throw them into that world and benefits from it. Fits with what Aleister said about how Touma and the rest are in the world of adults now that Aleister no longer lead Academy city. I think what Aleister means is that from now on they no longer part of Aleister plot and they have to take responsibility of their own actions.
Okay so basically
The final chapter of Daihasei Festival arc that has Mitori and Shokuhou picking up Dolly 2 didn't happen in the original version as the chapters were being released. It was only in the volume version this chapter was added.
Meaning originally Mitori would've been defeated by Kuroko and she would've been sent to prison, you could consider it a soft retcon.
Dolly backup only appears at the end of the arc with no foreshadowing. My friend said: "I don’t see how the second dolly could be part of the original script if the entire encounter leading up to it and the reveal itself was part of an editors addition."
Also Shokuhou learned about her existence because of Gensei's memories. It seems like a last minute addition.
So originally Mitori wouldn't have gotten such a bright and happy ending.
Also about the kids argument: Characters in Toaru very often struggle with the responsibility of others dying because of them, Accelerator, Mikoto, Mugino and Touma in NT9.
Kids in Toaru are never left off the hook for their actions it affects and informs how they act now. Taking another human life is still a horrid act regardless of age.