I kind of let the decision slide, as it was in character for her. Though now that I think about it, she may have had something else planned had the mercy kill not happened.
I think in the case of the General, he used the gun, knowing it was set to "kill," under the condition that he knew the General was going to regenerate. If it had been the last incarnation, the Doctor might not have pulled the trigger.
Didn't he say something like, "Death is like man-flu for Time Lords"?
If I had to guess, she had
something in mind for addressing the spiders, but that's based on knowing who the Doctor is and what....
Idle, but serious question: which pronoun is correct when discussing a trans woman's actions from before their transition?
Anyway, I base the aforementioned guess on knowing the Doctor's MO and history. On the other hand 11 was about to lobotomize the space whale to end its pain and save the colony ship in its back. The Doctor is no stranger to violence, or killing, or mercy. The War Doctor was prepared to whip out two races to spare the rest of space and time.
Robertson pulled the trigger out of hate and fear and ego. I won't say that he did the right thing for the wrong reasons, but maybe he did the "least bad thing" for the wrong reasons.
Maybe there was something that got left on the editing room floor that would have fleshed this out and addressed the ones trapped in the panic room.