I agree in general, as in the OP, but I don’t think this specific argument is valid. It’s possible that if the guy sees someone pointing a gun at him that he would drop his rather than get shot himself.
And from the officer’s point of view, she might have regarded getting his attention as a positive. Yes, it meant that he shot at her, but he was going to be shooting at someone anyway, and shooting at her means that he’s not shooting at someone else. She might have taken her job of protecting civilians seriously enough that she considered that worthwhile, and she might have been wearing some level of armor and was thus less vulnerable than other targets.
The policeman did her best in a difficult situation. She made no major mistake. I will not second-guess her.
I will. As has been pointed out, this was an individual who had already shot people. At that point in time, she should have shot him without saying a word.
You are ignorant of current police training regarding active shooters. Nothing you posted is a valid response to an active shooter. If that officer states what you wrote in her post use of force interview she’s going to be up to her neck in it.
This was not just a man with a gun situation. This was an active shooter, a man with a gun already firing. The response to both scenarios are different. As I posted before it’s easy to Monday morning quarter back, but she did respond in a manner that is not appropriate to engaging active shooters.
We see what you did there, ECG. 
As for the OP, I’m with those who say “not enough information in this case”. It may have been the right thing to do or it might not. I’d have to hear the officer’s side of the story before chastising her.
I’m not chastising her but I am critiquing her, which is exactly what her trainers and supervisors are going to do after going over her post use of force interviews. (Usually an officer does not write their own report when they’ve used or attempted to use lethal force, and/or when they are a victim. They give an interview to an investigating officer who writes a report regarding the incident).
Using the given information, had the gunman not been firing but simply holding a weapon her response may have been appropriate. But that wasn’t the case.
She fell back on training for one scenario instead of the other. But in a split second decision that can happen. In full disclosure there has been a time when I pulled out OC when I should have withdrawn my baton. That decision was critiqued later on, as it should have been.
We’re simply critiquing that officers action and it wasn’t correct for the situation.
Do supervisors usually give use of force critiques without knowing critical facts like what the officer observed before acting?
No, that’s what he post use of force interview is for. These critiques usually don’t come out for weeks or even months afterwards. Lots of things to consider including actions prior, officer/subject matter, training, department SOP, and so on.
Mentally ill but treatable = unfortunate, but if you go off for the rails like old yeller I don’t mind you being put down and prioritizing the safety of the sane.
Exactly. Sorry, but if you’re firing a gun at people, especially in public, then you get no sympathy from me if the police decide to shoot first and ask questions much later at the coronial inquest.