Now that we're back on topic...
I think what people like this "truther" experience is reminiscent of the "Delirium" from the old World of Darkness games. They try to disbelieve that such evil can happen without a massive conspiracy, and that might help explain how we get situations just like this one. Let's be clear: from the 9/11 truthers and the Holocaust deniers, we learned that no amount of evidence will convince them they are wrong. We could make them help clean up the crime scenes, and they would still deny it was real.
All because they are too horrified by the alternative. That it not only can happen, but it DID. So I do not begrudge them, but I don't hesitate to call bullshit on them either. Every now and again, you can guide them away from their self-imposed delirium, but you have to be firm and the chance is quite slim indeed.
I think that's part of it as well. People don't want to believe that anyone could barge into an Elementary school and blow away a bunch of adorable, innocent, helpless moppets with an assault riffle. No one wants to believe that, even in our modern era of high-tech espionage, that some info could fall through the cracks & lead to weirdos destroying a huge NY landmark with planes (killing thousands in the process). No one wants to believe that an oppressive government could just ship over 12 million people to prison camps for the sole purpose of getting murdered.
Of course, some of it is about conflicting ideology (6 million Jews murdered in Nazi death camps can put a big damper on the 'appeal' of the 'Evil Jew'/'White-Gentile Supremacy' stuff). That said, I do believe a lot of this is due to folks not wanting to accept the darker aspects of reality....that there are creeps who can & would blast-away small children....that even the most sophisticated spy agencies can miss clues of a bunch of medieval-minded goon's plot to commit the Mother of all Terrorist Attacks....that a lone weirdo could get a bunch of SWAT Team armor, bust into a movie theater & blow-away folks....that you can't ever be 100% safe in the alleged safety & shelter of a school, house of worship, movie theater, mall, etc.
You have this mixed with certain bigotries or obsessions (guns, etc.) and you end up with a bunch of weirdos who fall into goofy conspiracy theories. It doesn't help that the govt. itself doesn't away do the right thing & there's corrupted corporate types messing around behind the scenes...creating a "cry wolf" situation for the government in question.