There are a number of theories about why Adam Lanza killed 20 young children and 6 adults in Newtown, Connecticut ("As nation mourns, investigators try to figure out what led to tragedy in Newtown, Conn"): 

Federal investigators planned to visit dozens of shooting ranges and gun stores across Connecticut Sunday, attempting to figure out what led smart but painfully awkward 20-year-old Adam Lanza to murder 26 children and adults at Sandy Hook Elementary School in Newtown... 

Some people say lax gun laws are to blame; former Gov. Mike Huckabee said it was because we took God out of our public schools, while some others blame the lack of mental health treatment, and a "gun culture" promoted by violent movies, TV, and video games.  

My own feeling is that at least to some degree, many of the reasons listed above are to blame. As I wrote before in "The NRA gave him the gun and Hollywood gave him the reason: James Holmes and the Aurora Theater Shooting": 

Considering the destructive and dysfunctional message Hollywood pimps for a cheap buck, maybe it's time Congress imposed a sin tax on Hollywood movies to make up for the cost to the rest of society. There is no reason a sin tax should just apply to gambling, alcohol, and tobacco when Hollywood trash is just as dangerous.

As far as "God" not being in public schools, that answer is downright silly. The better answer is that the modern neoliberal state has destroyed any sense of community, which organized religion used to represent, and in its place, we are now only a collection of individuals. As Margaret Thatcher wrote, "there is no society anymore, only individuals". 

Mr. Neil Clark best made this point with his recent article "Get used to Cumbria-style killings in neoliberal UK: The egotistic culture of free market capitalism is to blame":

By encouraging us to be selfish and ultra-competitive, neo-liberalism destroys social cohesion. Other people are seen as threats and rivals, and not as potential comrades. In a neoliberal society it's much harder to make deep and lasting friendships and for families to stay together. Trust - that most essential element for building meaningful relationships - is eroded.

One of the odder things I have noticed is the convergence of Hollywood post-apocalyptic movies and Evangelical Armageddon theology. Two antagonistic groups, Evangelical Christians and Hollywood, have found common ground and are indulging in End of the World fantasy. And as a result, there are many Americans, Christians and non-believers alike, who are building an arsenal, not for basic self-defense, but for the End of the World. To them, there should be no limits whatsoever to the Second Amendment, not because they want to stop a burglar or mugger, but because they are waiting to fight the Antichrist or the next Civil War. 

Interestingly enough, the great conservative icon Ronald Reagan, supported gun control ("Ronald Reagan supported gun control, therefore your argument is INVALID")

As governor of California, Ronald Reagan signed the Mulford Act, which prohibited the carrying of firearms on your person, in your vehicle, and in any public place or on the street, and he also signed off on a 15-day waiting period for firearm purchases. After leaving the presidency, he supported the passage of the Brady bill...

His own words:

With the right to bear arms comes a great responsibility to use caution and common sense on handgun purchases. And it's just plain common sense that there be a waiting period to allow local law-enforcement officials to conduct background checks on those who wish to purchase handguns."

Also, see "Gun Control, Or Reloading Morality?" by Gareth Price: 

Reagan and Thatcher's neoliberal ideology inevitably entails the inculcation of a widespread amorality and misanthropy among the population. With Friday's events -- and the other half-a-dozen similar incidents this year alone -- it should be no surprise (though no less of a tragedy) when psychopathy comes home to roost.

You can listen to my podcast I did on this topic here "Newtown School Shooting: What can be done?". One point that I make in my radio show is that I think the modern decline of monogamy, and rise of both polygamy and serial monogamy, combined with easy access to weapons, is the single biggest cause, or proximate cause, of recent gun rampages (see "monogamy reduces crime").