New information suggests (as in ... not yet confirmed) that Mrs. Lanza had reached the end of her ability to handle her son, and had applied to a court (probate, presumably) for a writ of conservatorship preparatory to having him involuntarily institutionalized. The son/shooter apparently found out about this. The theory is that he decided his mother cared more about the kids in the school than she did about him, and so he set out to get revenge.
It is, at this stage, only a theory, but IMHO it's probably a good one if it can be confirmed that Nancy had applied for conservatorship (or that her son might have had reason to believe she had). From his perspective, he would have felt betrayed by his mother. He would have (probably) felt he had no way out. If he did nothing, he would be institutionalized. If he acted out in some angry but insignificant way ... he would still be institutionalized. He was (in his mind) looking at a dead-end street no matter what he did, so why NOT take out his betraying mother and as many of the kids he felt had replaced him in her affections ... and then just off himself once the game reached checkmate?
I'm thinking it makes sense.
And IF it holds up, it means all this talk about gun regulation doesn't mean diddly. What do we have here? We have a freakin' genius who wants to destroy the kids in the school. Let's remember that Dylan Klebold and Eric Harris set two 20-pound propane bombs at Columbine. Miraculously, they didn't go off, or (according to the investigators at the scene) the death toll would have been in the hundreds rather than in the teens. So if Adam Lanza, boy genius who hates his mother and the school, can't use a gun to extract his revenge ... he's certainly smart enough to construct a bomb that WILL detonate on command.
Once again -- the problem is not the guns, the problem is the people who would attack a school and kill small children. If this theory holds up, I would say the problem is that Nancy Lanza waited too long to do something about Adam, and didn't act decisively (or circumspectly) enough when she did finally decide to do something.