First, every police officer or security officer eligible to carry a weapon should be carrying a taser of some kind. It is crazy that in 2017 many if not most cops still only carry handguns, given that they are far more likely to need to subdue someone as opposed to shooting and killing them.
Second, if you are a cop and call in a report of somebody with a weapon, I'm pretty sure help will arrive pretty quickly--and meantime, there are things that can be done to mitigate the situation for a few minutes. You can talk to the disturbed person, you can clear the area, you can back away; you can, in short, stall until help arrives--but of course help would just be more cops inclined to pull out their guns.
Third, I find it incomprehensible that police officers are not trained to shoot a potential assailant in the leg. I'm PRETTY sure that shooting a knife-carrier in the leg would end the threat, as a bullet in the leg TENDS to be pretty painful and incapacitating. Why wouldn't this be a part of our police training, in a situation like this. I'm not suggesting that cops in a gunfight try to shoot an assailant in the leg; I'm suggesting if some disturbed person is standing 12 feet in front of you with a knife, shooting in the leg certainly seems like a viable option that would involve no loss of life. You know what's also incomprehensible: Let's say there were three cops and one disturbed kid with a knife (of some kind). If I were one of the three cops, I would be inclined to suggest to my fellow officers that we tackle the guy and subdue him--perhaps after one cop distracts him or somesuch. Would one or more of the officers risk getting stabbed? Yea, maybe--but they'd also be protecting the life of the mentally disturbed person. Alas, our cops do not think that way.
Fourth, cops deal with mental troubled people every day. Indeed, a large number of individual disturbances they deal with are people with mental problems. If a young person is standing in the public place in ratty clothes and no shoes and is holding a knife, it doesn't take a Ph.D. to know that he's probably having a mental episode. In fact, that is almost always the case. Rational people do not take weapons out in public.
Fifth, and I alluded to this above, being a police officer MEANS that you will probably experience threats. That's part of the deal of being a cop. And yet we have loads of cops who are absolutely terrified of getting hurt and this is a big reason why so many police needlessly shoot and kill people. I should think that a HUGE part of police training would focus on subduing people without loss of life, since incidents like this are very common. But apparently this isn't the case. If somebody has a weapon, cops tend to pull out their guns and shoot. Almost anybody with a gun can shoot somebody; I should think the point of having a well-trained police force is to try and avoid shooting unless it is absolutely necessary. I think our cops generally are trained not to take any chances, which, again, is risk averse strategy that doesn't require much training. This points up how unsophisticated our police training is, in my opinion. Cops would respond to all this by saying that shooting someone is the most efficient way to protect the public. Protecting the public is crucial--but what's being missed is that a disturbed person with a modest knife is the public too. He or she deserves to get mental help, not be shot and killed.
It is weird how primitive America is in so many ways, still. We have a reputation for being an "advanced" country and that isn't even remotely the case.