People don't want to believe one random kid could cause so much tragedy.
Every major event ends up trailing behind it a fog of conspiracy theories like the tail of comet, and the Sandy Hook elementary school massacre is no different.
There are two main categories of conspiracy theory at work, and both of them have the same ending: Obama takes away all our rights.
According to the first category, the whole thing was a staged event. A false flag operation (conspiracy theorists love the phrase "false flag") designed to terrorize the populace into demanding succor from the government.
Unfortunately for those who preach this particular category, most of their facts are just plain wrong. For example, this particular image has been making the Twitter rounds like crazy. I count at least seven major factual errors, but that kind of thing doesn't stop people believing it.
The remainder of their facts are more easily explained by the commonplace phenomena whereby people who experience a tragic event prove to be terrible eyewitness reporters. In the chaotic hours which followed the shooting, there was a lot of confusion about who had done what and when and how and why and what the heck really happened.
Personally, I feel that just because the facts got sorted out later by professional observers (i.e. police officers and other first responders) that doesn't necessarily imply a vast cover-up at work.
You can never truly debunk this sort of conspiracy, because they will claim that either A) you have bought into the party line, or B) it's a huge cover-up, or C) you're a tool of the Illuminati trying to propagate confusion and disbelief among the true believers. (I get accused of being a tool of the Illuminati on a regular basis. Call it a job hazard.)
I guess it's easier to believe that the whole thing was faked, or that it was a deliberate military action, rather than believing the truth: that one crazy young man can destroy so many lives for no legitimate reason.
In a sub-branch of this category, the Sandy Hook massacre happened because Obama's dark cabal used black magic to make it happen. This belief is being promoted by Alex Jones at InfoWars. He draws a crazy web of connections between the Aurora massacre, Sandy Hook, and the most recent Batman movie.
In the second category, everything happened as per the conventional mainstream belief. However, Obama is leveraging this tragedy in order to take all our guns and institute martial law and basically kick off the apocalypse/civil war that the preppers have been waiting for. This one is impossible to debunk because it's pure theory. Until the jackbooted thugs institute door-to-door searches for all weaponry on the citizens, I guess we'll just have to put it in the "Pending" file.