Why do mass murder conspiracy theories flourish, and how can I help stop them?

If you're on Facebook, or have conspiranoid coworkers, or live anywhere other than beneath a rock, you've probably heard at least one of the following conspiracy theories about the Aurora and/or Newtown shootings:

•  James Holmes and Adam Lanza were mind controlled assassins, programmed to kill large numbers of people so that Obama/Communists/Rothschilds/whoever can push through restrictive legislation, damage the reputations of certain citizens, declare martial law, psychologically destabilize Americans, make blood sacrifices to the Devil, and/or distract us from "more important" things that are happening. Infowars promoted these theories, here and here and here and here. More recently, a fellow prison detainee claimed Holmes blames his therapist for programming him to be a killer, but there's no way of knowing at this time if Holmes actually said such a thing or not (IMO, it seems unlikely that Holmes would be left alone with another detainee for four hours, as Steven Unruh says). Nonetheless, the story was picked up by Infowars (here) and by The Huffington Post (here). Note that while the tone of the HuffPo article is justifiably skeptical, the Infowars article decries the mainstream media's neglect of this "shocking" story and uncritically takes Unruh's word for everything.
• James Holmes' father, Robert, was scheduled to testify at a Senate hearing on the Libor interest rate scandal at the time of the Aurora massacre. Clearly, the bankers called in New World Order mind controllers to program his son to become a killer so that his invaluable testimony would never be heard. An Infowars forum user promoted this theory here.
• Adam Lanza's father, Peter, was also scheduled to testify on the Libor banking scandal (the same Infowars forum user promoted this theory, here).

The last two theories are quite similar to one that surfaced after nanny Yoselyn Ortega murdered her two charges, the son and daughter of CNBC executive Kevin Krim. Several conspiracy sites, like this one, immediately adopted the theory that someone had programmed or otherwise manipulated Ortega into a double homicide and a suicide attempt because the CNBC website had published a report on a $43 trillion lawsuit that has been filed against "banksters" and a laundry list of financial entities. The theorists noted that the article was removed almost as soon as it appeared, meaning Krim may have recognized his children's murder as a message to continue keeping a lid on the lawsuit. However, this "report" turned out to be a press release from Spire, the law firm behind the lawsuit. Chances are good that it was posted on CNBC by accident, then removed when someone realized it wasn't a legit news story. In all likelihood, because it so incredibly broad and vague, the lawsuit will be tossed out of court. I doubt the banking establishment is at all concerned about it. Furthermore, when Ortega recovered from her self-inflicted wounds and spoke with investigators a week after the murders, she made it clear that her crime was motivated by her resentment over being allegedly treated like a maid by the Krims. This is starkly at odds with one blogger's confident assertion: "That is, a priori, and without any detailed detective work, I feel there is roughly a 9 in 10 chance that Lucia and Leo have been murdered by rogue elements of the United States government."
On closer examination, the shooting theories are equally groundless. Robert Holmes was employed as an anti-fraud specialist for FICO, the credit-scoring company, but he had no connection to the Libor affair. There was no Senate hearing scheduled, anyway.
Adam Lanza's father has no link to the scandal, either. Max Keiser.com and Occupy Corporatism, among other alt media outlets, have been doing their best to debunk this bit of misinfo. They allege that it originated with Sorcha Faal, the same peculiar conspiracy-monger who was behind one of the least credible conspiracy theories about Michael Jackson's death.
No actual evidence has emerged to indicate that either James Holmes or Adam Lanza were in any way influenced, coerced, or brainwashed into committing their crimes.

So misinformation is the backbone of these theories. Another part of the problem is that the average person has little to no knowledge of criminology. That's probably a good thing - knowing how criminals behave isn't exactly a marketable skill. But when it comes to creating conspiracy theories, it's going to seriously hamper your ability to put together a coherent and plausible narrative. One thing I hear a lot in connection to mass murder is that the killer looked blank, glassy-eyed, or robotic. This is presented as proof that the killer was under some form of mind control. In reality, if you talked to and/or observed people with Intermittent Rage Disorder or Antisocial Personality Disorder (to give just two examples), you would quickly learn that these people can and do appear poker-faced when carrying out crimes that would turn other people's stomachs. The survivors of their crimes can tell you this. A man with Intermittent Rage Disorder once told me he feels paradoxically calm and focused during his rages.
If you review mass murders of the past, you'll find that the killers' motivations were generally not political in nature (Anders Breivik would be a notable exception). For them, it was personal. They were, for the most part, emotionally or mentally disturbed individuals who killed in retaliation for real or perceived slights. Some did it for pure enjoyment (I believe Eric Harris falls into this category; Dylan Klebold went along for the ride, and seemingly remained in denial right up to the time of his death).

So how can you help prevent the spread of shoddy, factually-challenged conspiracy theories? Before turning an online conspiracy theory into copypasta, do yourself the following favours:

1.  Take 5 minutes out of your day to hit the search engines of your choice and make sure the information is accurate (odds are, it isn't, and you've just saved yourself some time and energy). Remember that anyone can be wrong, so information isn't necessarily reliable just because it has been repeated by a certain source.
2. Apply common sense to the situation. For instance, if Adam Lanza is a Manchurian Candidate, who programmed him? He was homeschooled, so it couldn't be those godless public school Commie teachers. He didn't have a job, so it couldn't be his boss. His mom wasn't exactly up to New World Order mind controller standards. Besides, would some shadowy government entity spend months or years grooming an unstable and unreliable young man to become an assassin, when mercenaries and unscrupulous criminals and CIA agents can be hired on the cheap to accomplish the same task more efficiently?
3. Apply Occam's Razor for good measure. Is it more likely that a sadistic or deranged person decided to commit a crime, or is it more likely that a government-funded Svengali flawlessly conditioned him to commit a crime? If a liquor store is robbed, do you automatically assume that a mind-controlled patsy did it, or do you assume that some cash-hungry creep did it?