She was moving aggressively at the girl and swinging the knife around. I'm not saying she wasn't going to stab her, but that I personally doubt it. She doesn't appear to stab the girl who gets pushed/kicked to the ground as best as I can tell from the footage, and all I can find in articles is a statement that she "tried" to stab the girl on the ground. Do you have other information? My doubt comes from the fact that if you're actually trying to stab someone at close range, they're pretty much going to get stabbed somewhere. Also from personal experience with violence, where producing a weapon and even menacing and threatening with it has more been about posturing. I also am just a bit incredulous that anyone could be stupid enough to actually stab someone in front of a cop in broad daylight, rather than just dumb enough to think that they could get away with threatening.
As I said before though, none of that is relevant to the situation as it was though. The cop can't risk someone else's life on the possibility that someone is only 80% retarded rather than 100%, and he did not have time to do anything else or more.
From looking around, it actually seems there's pretty solid consensus that it was justified. Hell, even youtube comments are largely in agreement. The biggest difference in reactions from across the political divide seems to be the language used with some saying "What a tragedy, what terrible circumstances, and let's fix the foster care system! But, cop did the right thing." (Which seems a bit naive and simplistic compared with "Cop absolutely did right, but no one will admit it, and this will cause riots!" The small number of people objecting seem to be saying he could have used another option but verbal was tried and failed, he was too far away for physical intervention and even if he'd had his taser out and pointed it's not reliable or quick enough stopping people for pink girls safety.