It is now being reported that the gunman had Asperger's syndrome.
http://www.nytimes.com/2012/12/15/nyregion/adam-lanza-an-enigma-who-is-now-identified-as-a-mass-killer.html?smid=tw-share&_r=1&
My kid brother (now a young adult) has Asperger's and I've experienced his mood swings and unpredictable reactions first hand many times. In fact, that's why I have (finally!) convinced my mum that he needs to leave home and go into sheltered housing. He will never integrate properly into society but stands a better chance in the right environment, which isn't at home.
I fear that one day I will get a call to say that be has really hurt her, or worse
As I have explained to her, I can't always drop everything and make the 500 mile round trip when he's getting out of hand
Although he has improved after the last "chat" I had with him 
My point is, though, that it is a very unpredictable condition and someone suffering it can "flip" for the tiniest of things! For him it can be as silly as it's Raining today!
The following came into my facebook today from an organisation called 'Always-Unique-Totally-Intelligent-Sometimes-Mysterious' an American site on those with ASD and their families, and linked to the following.
IME those with Aspergers and not so agressive but some with high end Autism can be and can 'turn' for no apparent to the non autistic person, not so sure I agree to everything said but I guess they were going to react....
I quote: While an official has said that the 20-year-old gunman in the Connecticut school shooting had Asperger's syndrome, experts say there is no connection between the disorder and violence.
Asperger's is a mild form of autism often characterized by social awkwardness.
"There really is no clear association between Asperger's and violent behavior," said psychologist Elizabeth Laugeson, an assistant clinical professor at the University of California, Los Angeles.
Little is known about Adam Lanza, identified by police as the shooter in the Friday massacre at a Newtown, Conn., elementary school. He fatally shot his mother before going to the school and killing 20 young children, six adults and himself, authorities said.
A law enforcement official, speaking on condition of anonymity because the person was not authorized to discuss the unfolding investigation, said Lanza had been diagnosed with Asperger's.
High school classmates and others have described him as bright but painfully shy, anxious and a loner. Those kinds of symptoms are consistent with Asperger's, said psychologist Eric Butter of Nationwide Children's Hospital in Columbus, Ohio, who treats autism, including Asperger's, but has no knowledge of Lanza's case.
Research suggests people with autism do have a higher rate of aggressive behavior — outbursts, shoving or pushing or angry shouting — than the general population, he said.
"But we are not talking about the kind of planned and intentional type of violence we have seen at Newtown," he said in an email.
"These types of tragedies have occurred at the hands of individuals with many different types of personalities and psychological profiles," he added.
Autism is a developmental disorder that can range from mild to severe. Asperger's generally is thought of as a mild form. Both autism and Asperger's can be characterized by poor social skills, repetitive behavior or interests and problems communicating. Unlike classic autism, Asperger's does not typically involve delays in mental development or speech.
Experts say those with autism and related disorders are sometimes diagnosed with other mental health problems, such as depression, anxiety, bipolar disorder or obsessive-compulsive disorder.
"I think it's far more likely that what happened may have more to do with some other kind of mental health condition like depression or anxiety rather than Asperger's," Laugeson said.
She said those with Asperger's tend to focus on rules and be very law-abiding.
"There's something more to this," she said. "We just don't know what that is yet."
After much debate, the term Asperger's is being dropped from the diagnostic manual used by the nation's psychiatrists. In changes approved earlier this month, Asperger's will be incorporated under the umbrella term "autism spectrum disorder" for all the ranges of autism.
End of quote. I will be interested to see what happens in this country, oh and for information the latest stats for a child with ASD is 1 in 80............