I pretty much agree with Raprap's take here. These events are exceedingly rare but exotic, so it's human nature to overreact to them and try to eliminate them because accepting that there will always be a risk feels like defeatism to many. The simplistic reaction is always to try to prevent
(absolutely, if possible) such rare horrible events. The part that is counter-intuitive to most people is that preventative security is nearly impossible, and reactive security and culture modification are the best we can do to reduce the odds of this happening again and how we react really matters the most (our security is not built on preventative security it is anchored on reactive security and the threat of what society will do in reaction
to the violation), but if you don't accept the reality that there is some degree of risk for this event that is inevitable, and that there are diminishing returns as you go about trying to reduce the risk, and that there is always a cost of some sort to doing so then the position is a mere fantasy. Reality is simply not as amenable to having complex systemic issues resolved by intuitive looking tweaks. Reality is a lot more chaotic and less controllable than this.
These events can't be eliminated, and banning guns will have only a small effect on how many people die this way (it's not all that much more than the government has killed when trying to take people's guns away in places like Waco and Ruby Ridge to put it into perspective). I still support the general idea of gun control and more regulation but it honestly doesn't make much as much of a difference as the public's obsession (to the point of fetishization) with mass murderers and that we let these things become cultural events is something that poses a much greater risk (though obviously not sating curiosity is a tougher tradeoff than just not having guns) yet we aren't willing to stop rubbernecking.
In other words I think the fact that we are talking about it has a greater impact on its likelihood of recurring than banning guns would. The limiting factor is nearly always the availability of disturbed individuals willing to throw their lives away for fame. That we grant these killers fame and notoriety is probably the single biggest thing we change. Norway's reaction was healthier. Not trying to reflexively let the killer change them, trying to keep the killer on the ground, not a martyr and not an obsession. A healthy self-awareness of the impact of their reaction is what they have an
If I had to advocate one single thing here it would be that the media avoid naming the killer and didn't pore over his every pore (I will already never be able to forget this face, having seen it and read it dissected so) trying to outdo themselves for the rubbernecking society. Denying these killers their fame and notoriety will do more to reduce these cases than gun control will.
I still think Gun control can help (especially for mass murder when it comes to large magazines and infantry weapons), mainly because the next best weapons for these killings are nowhere near as good (witness that he tried bombs, but nobody died from them) if you intend to get out alive. But that all can change very rapidly, and how a culture treats it makes a big difference.
The weapon of choice for mass murder in the last ten years has been overwhelmingly the suicide bomb, the biggest reasons this is less prevalent in America is not due to superior "bomb control" but due to cultural differences in how death and suicide is viewed and how few people are willing to "martyr themselves" in our culture.
My problem with most gun control advocacy is that it's sometimes like trying to just wish away bad things. Bloomberg's big soda fight will save more lives, just not in ways we care about as emotionally and not in ways we are as prone to become so irrational about.
My main argument is that society does not view these singularly horrible events rationally. Gun control can help us a bit but in most people's minds this is expressed as "if we just could ban these things we'd stop all these senseless killings" when in reality the rate at which people are killed is influenced by culture and society much more than by the available toolset. I'd love to see greater restriction of particularly dangerous tools (especially weapons that can kill many people at once, as their utility as defensive weapons is even more negligible than guns in general) for no reason other than there's not a huge downside to trying that but getting society to understand and react to risk better would be a much better thing than if there were suddenly no guns tomorrow. This is the most peaceful secure time in human history, it is so not because of the regulation of weapons but because of the evolution of society (like globalism, economic contagion is the world's greatest peacemaker right now). Complex systemic solutions aren't usually solved by simple solutions but our brains are wired to try to look for simplistic solutions that can fit a simplistic narrative arc and in the process the frenzied reaction is, in my opinion, about as dangerous to us as the guns are.