I don't understand why people want Ishida to kill Mutsuki and that it'd be an asspull otherwise. Mutsuki is supposed to be a parallel to Kaneki, and it's only likely that she'll murder her torturer anyways. All this Mutsuki hate seems to be just people obsessed with Kaneki or the Anteiku members.
No, personally I think it'd make more sense if Mutsuki died. She was being turned into a torso anyway, and if I'm not mistaken, that's how Torso got his name.
If she survives and kills her captor, then it's just a rehash of Kaneki and I cannot see it leading to something meaningful. I mean, 1 Kaneki is enough. Also, her dying would make a nice statement that some people can get through hard shit (Kaneki), and others are just submerged by it (Mutsuki). This will also cause nice character development for Urie and make for a nice fight and aftermath when all's said and done (I.e., They won, but lost as well).
Tl;dr, Mutsuki dying makes more sense than being alive, and impacts the story and characters in a much more meaningful way, as well as the audience.
I agree of course we will have complainers who will say Ishida is bring to cruel, heartless or whatever. Of course it will be a cruel death that may be much for some people to handle but the world is cruel and bad things happen. In TG part one it always seemed like "the world is wrong." Was a major part of the TG world that bad things happen to good people because it's the way of the world. Either way Mutsuki alive or dead it will be the end for them. But the latter would be more meaningful in my opinion just to more reflect on how messed up the world is and that everyone does not survive being traumatized and tortured. If Mutsuki is alive they will definitely be another Kaneki probably a more cruel one and that is just very "mmh" to me.
If people really care about Mutsuki why would they prefer the character to be ruined in that way? Then a more powerful meaning and better legacy left in death.