Update: great op-ed about this subject. Thanks to reader, Reader, for the tip.
The Newtown shooter was said to have Asperger's Syndrome which is on the spectrum of autism. These people are considered the high-functioning end of that spectrum (even as they face very real and distinct challenges.)
One thing to make clear - Asperger's is NOT mental illness. It is a neurobiological condition. While some mental illness, like schziophrenia, does have a biological basis, Asperger's is not defined in that category.
After saying that, based on my own personal experience with Asperger's, I suspect the shooter DID have mental health issues in addition to his Asperger's. It may have been depression, anxiety or bio-polar.
From news reports, it seems clear that he was did have a diagnosis of Asperger's and his high school knew about it. His mother had to come to the school sometimes if he was having issues. But all reports were that he kept to himself, that staff watched over him, the other kids were generally nice to him and that there was no evidence he was bullied.
(Indeed, Aspies are far more likely to be bullied or be victims of crime than be violent. They can get easily frustrated but that doesn't usually play out in acts against others.)
The principal at his high school, being careful in what he said, did mentioned that perhaps after leaving the cocoon of protection in high school, the shooter did not feel as comfortable out in the greater world. (He had, at some point, attended some college classes.)
Aspies generally do not like loud noises so I was quite surprised to read that his mother taught him to shoot (thinking it would teach him discipline and responsibility).
What is also clear is that we have cut way back on mental health services to the point where many mental health care professionals are almost frantic with worry. Washington State apparently has one of the worst mental health systems in the country due to lack of funding.