First of all, we have ample evidence that gun laws do have a positive effect. States which have enacted them show a decrease in gun-related crime. Of course some people will ignore the law, but it's simply not true that the laws don't keep the guns out of the hands of some criminals. We have data on this; we don't need to guess.
As for the "arm everyone" argument, there is much to say against it. First of all, there's the false assumption that lots of people with guns in an active-shooter situation will reduce the death-toll. Untrained, panicked people are very likely to kill innocents. Can you imagine if a bunch of armed people in a darkened, frenzied movie theater (where some shootings have taken place) tried to shoot back? Plus, if multiple people have guns, how do you even know who to shoot? Am I shooting the shooter, or another concerned citizen? For that matter, how do the cops differentiate between "mass murderer" and "person trying to help" when they get to the scene? One former military person who was armed and was at the Oregon shooting kept his gun holstered for precisely this reason--he didn't want to be killed by SWAT when they showed up.
But, here's the biggest problem with the "Only a good guy with a gun" argument. It's missing the larger reality. All of this rhetoric says nothing about what arming more people does to our safety on the 99.99999% of the days in which we are not faced with an attempted mass-murder. The more guns there are in the room at any time, the more likely one is to be used in an altercation, or to be discharged by accident. My standard is not "how likely is my child to die at school if a mass-murderer shows up?" but "how likely is my child to die at school on any given day?" Even if more guns would cut down on the mass murders (a dubious claim), they would almost certainly increase other deaths. That seems blatantly obvious to me.
I've used this metaphor before--I can imagine a situation in which wearing a seat belt makes it more likely that I'll die (driving into water, for example). But, on any given car trip, wearing the seat belt makes it more likely that I'll live. Focussing only on the rare exceptions only serves to hide the larger, more common truth.
If our goal is to survive a day, then I'm pretty sure that giving more people more guns is not the way to go about it.