There have been several news stories reporting that people said that Adam Lanza didn’t feel any physical pain. For example, the AP reported :
Richard Novia, the school district's head of security until 2008, who also served as adviser for the school technology club, said Lanza clearly "had some disabilities."
"If that boy would've burned himself, he would not have known it or felt it physically," Novia said in a phone interview. "It was my job to pay close attention to that."
It sounds to me like Adam had an extremely rare condition called congenital analgesia, which is actually a very dangerous thing to have because people with that condition will injure themselves without knowing it.
I think it’s interesting that in this article, about a man who suffers from the disease, the guy explains how his brother, who also had the disease, committed suicide.
So if Adam Lanza had congenital analgesia, there was a lot more going on than a mere case of a kid with Asperger’s Syndrome.
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It's also interesting to note that Ryan told police he hadn't seen his brother since 2010. Which probably means he also hadn't seen his mom since 2010. And it's pretty unusual for someone not to visit his mother and little brother for two years, given that they don't live so far from each other. If true, this probably means they were both pretty messed up and Ryan wanted nothing to do with them.
It's also not uncommon for people with mentally disabled siblings to severely resent their parents for giving all the attention to the sibling with the disability. That may have had something to do with it.