I have been thinking about the prevention aspect of this. And I'm sad to report that we will never have 100% prevention.
The problem is not the availability of weapons.We may be more efficient about it now, but humans have always had the capacity to kill, even when the only weapons available were our fists and rocks. Discussions about removing the weapons from society are moot. Several nations have tried and failed. 100% removal is not possible and there is some validity to the argument that it leaves the law-abiding even more vulnerable. It is also an impossible task. Criminals will keep their weapons no matter what law we pass - they are the tools of their trade. And the number of law-abiding citizens who will give up their weapons is highly debatable. Some will. Some will passively refuse, convincing themselves that since they are not a threat, their particular guns should not be taken - making them criminals on paper. Some will actively resist enforcement. The number of dead on both sides in those scenarios could easily outstrip the numbers of dead to this point at the hands of serial and spree killers. We cannot change this part: the guns, knives, poisons, fists and rocks are here. They will always be here.
The problem is not books, movies, TV or video games either. Depictions of violence have always been part of every society's story telling and entertainment. From the earliest bushmen telling the story of how a young hunter saved the tribe by killing a raiding tribe's scout to the slow-motion carnage of ridding the Earth of mutants, aliens and zombies. Violence is within all of us. And before you say you are immune to the urges, ask yourself what you would have done if you had a chance to confront any of the latest shooters mid-spree. I'll make it easier for you. Ask yourself what you would do to protect your child. Knowing that the violence is inside us, but we aren't all serial killers or spree-murderers proves that our total immersion in violence from all sides is not slowly nudging us all to become murderous. It is an easy reaction to blame the images around us, but it doesn't wash because with a world population of 6,973,738,433 (and counting), the incidences of murder and mass murder are still rare enough to make the news. My argument here is is that if the violence around us is the source of unleashing the violence within us, we would soon wipe ourselves out and societies would collapse. (not that I don't think we are headed that way eventually for much different reasons).
The problem cannot be solved by increasing security. Ask anyone who deals with security on a regular basis - policemen, insurance adjusters, the military - if security can ever be 100%. You can increase the difficulty of violent interaction, but only by degrees. The answer only approaches yes with complete and utter person to person isolation. Only when every human is locked away, unable to interact with any other human in any way will there be anything like 100% security and safety from each other. Note that we would still be vulnerable to lots of other things. So, short of complete interpersonal isolation, each increasing degree of interpersonal contact increases the chances of violence and decreases security.
The concept of hardening one target to violence by adding more and more security necessarily weakens all other nearby targets. After our home was broken into, we added more security. Our goal was simple. Make our house less of a target when compared to other houses. That puts the onus on both the neighbor and the criminal to respond. Smart neighborhoods band together and make the whole neighborhood less attractive to criminals. The same concepts apply to hardening our schools against gunmen. What about the mall? The stadium? The PARKS! Hardening targets only shifts the danger to new targets. And universal hardening imprisons the innocent. Anything short is not 100% prevention. The gunman at this school bypassed the locked doors by shooting out a window. It was harder for him to gain entrance, but not impossible.
So, the weapons are here and will remain so. The targets are impossible to make 100% safe. What is left.
The shooter in this case, and in almost all of the most recent cases, planned this event. He interacted with people during this planing. If even one person had seen the signs or detected a shift in behavior, perhaps this incident and others, could have been prevented in it's entirety. Please understand that this next bit should not be construed as blame - it is conjecture. The mother had hobbies, among them sport shooting. If there came a time when her son showed any tendency towards violent actions, what should have been the disposition of her personal firearms? Locked up securely? Certainly. Removed from the home entirely? Perhaps. I conjecture this only as an example of what i feel our ultimate path forward to be.
Recognition, prevention, treatment.
We used to house our mentally ill. Many of the institutions were cruel places - prisons for the mentally ill with abuse and neglect. Now we medicate them until they are stable and then return them to society. With poor to no followup systems in place, many are soon off their meds by choice, unfounded reasoning or poverty and right back into their illness. If persons who are at or near a mental break who are or reasonably could expect to be violent were monitored with the same attention we give to other know dangerous situations, how different would this problem be? After the fact we consistently find that the shooters were troubled, that there was a triggering stressor, that they had made threats to themselves or others. I'm sure there were just as many who were on the path to violence and gave no apparent warning signs. And I believe that is the true difficult task at hand.
But what we are going to get instead is feel-good legislation. The laws which will be proposed and probably passed in the wake of this last shooting will be reactionary and visceral. Most likely they will seek to eliminate the weapons. We tried that once. The Assault Weapons Ban of the 80s banned whole classes of firearms and limited magazine capacity. The result - the law abiding had fewer guns, but the number of guns in America did not go down. Crime in America did not go down. Net effect on murder - zero. But people felt better.
We don't need a visceral response/ We don't need an emotional response. We need a true reasoned response. We are not going to get it.
A more reasoned response might be to control weapons better - not by banning whole classes of them - shooters would just shift their interest to the unbanned classes. But what if we expanded the background checks to include the criminal and mental stability of everyone in a gun buyer's household, not just the buyer. No sales to wives of felons, (sorry Mr Liddy), no sales to parents or spouses of mentally ill without strict lockup requirements. (The current regulations for unannounced inspections linked to Class III ownership could be a model for that one.)
While we are controlling the weapons, we could harden the most precious of targets. Schools become prisons. Schoolyards get high masonry walls with concertina wire and guard towers. Buses are armored and get armed security guards. Go ahead laugh. I can see this happening.
Any person who threatens another in any way is institutionalized. Speech and demeanor become as microscopically scrutinized as though we are continuously going through security at the airport.
Let's not forget our culture of violence. Every book, TV show, movie, video game, newspaper, etc. becomes one of the few episodes of Leave It To Beaver or My Three Sons where there was not even the insinuation of discord.
And we'd still miss detecting most of the shooters until it was too late.
I don't know how to fix it. I just know that it sucks.
edit - I posted this before I read Les's posts above. My apologies if I seem to have covered much of the same ground.