One of the biggest problems in this venue of operations relates to the raw human emotion that invades and distorts the reality, of which stats are used on both sides of the emotional argument to prove not much, really. The complexity is immense, with no easy solution forthcoming by throwing money or more guns at the problem.
People who have lost touch with reality, and grasp their own inner torture, wanting to share this torment with others, will do just that. Their actions involve surprise, horrific violence against the unprepared and unarmed, to which some die before the threat is even known to exist. That's the nature of an ambush with firearms. Even involving the well trained and experienced, in ambushing attacks, some are wounded and even die, before proper immediate actions are engaged. The key is minimizing the damage as quickly as possible as we get out of the kill-zone (often involving our own delivery of surprising violence, accurately delivered).
There is not a government around the globe that can really prevent these kinds of attacks from ever happening--even under the most seriously controlled, fascist regimes. Israel, one of the most armed, prepared and experienced nations on this planet still suffer from terror attacks that kill its citizens.
Preventative measures can be installed with passive and active protective devices that make it harder for souls to elicit these kinds of behaviors, but such measures work only to the degree the populace is willing to relegate personal freedoms away to the government, and the degree to which the assailants are willing to sacrifice themselves in the process. Those less convicted may think twice from making attempt.
Well trained, experienced shooters on site, in multiple roving patrols, properly armed, with full-select fire carbines/long-guns can also minimize the damage done when such an active shooter unleashes death and mayhem.
Throwing guns at people, hoping they will rise the occasion, when under such dire duress they will simply default to what they know best, will offer more problems to the situation than is already occurring.
When gunfire starts erupting, and supersonic rounds are cracking the air at ear level, impacting all about and screaming of the dying fill the air, along with the smell, people not prepared or experienced in such contexts have a natural tendency to default to their instincts. Even the trained who have never experienced horrific human violence before, but have made up in their mind they will prevail, will own a moment of hesitation from this kind of horror before they will be able to engage.
A real problems in this venue of dealing with active shooters are the people who have never been shot at, and who have never had to aggress upon and kill active threats, yet are always talking about what to do, how to do it and what needs done, as if they have first hand experience doing as much.
People who spend a life time training with their guns, and own guns, and believe guns will keep them safe, but who have never been involved, directly in killing combat, never tested for real, are often the most secure, confident, and the most ignorant of their methods of approach in this environment. Wishful thinking and denial are serious operational errors that get people killed quickly.
This is a multifaceted problem that needs every avenue of approach addressed, the least effective being arming civilians, hoping they will help.
School buildings need to be built with security in mind. Security equipment that visually monitors areas of access need installed; metal detectors with detailed scans and armed guards experienced at managing active shooters need to man these access points. Roving patrols of heavily armed, experienced operators with full select fire weapons need to be placed in areas of high concentrations of people need in place--shopping malls, airports, train stations etc.
I have operated in regions of such as what I describe above, and I am personally okay with that, but, I am not sure if the general populace in the USA is ready for roving patrols of heavily armed guards of experienced operators walking in tandem down the halls of our shopping malls and outside government building and in our halls of elementary, secondary, and higher learning establishments.
My personal assessment in the USA is that there are already too many well meaning nut jobs armed to the teeth and loaded for bear who don't even know what a damn bear looks like, let alone can deal with an enraged Mama Grizzly barreling down on them to shoot--metaphorically speaking. Banning guns or arming more people with guns, is not the answer.