There was a good reason our ancestors built their houses with secure attic or basement rooms.
It is my strongly held belief that underlying all childhood rage is humiliation, and quite often that humiliation comes directly from the parents. I'm not making excuses for Lanza, but I cannot assume his mother was a wonderful parent that just happened to have a child who was destined from birth to be a sociopathic killer. This was a family with multiple issues, and I would bet that an in depth analysis of how Lanza was raised, and the issues the parents brought to the family, would identify multiple contributing reasons as to why this happened.
Regarding the 13 year old child of the ‘writer’ Lisa Long, it is highly informative (IMHO) that she takes the time to write about this, letting us all know of the ‘cross she bears’, rather than spending all of her waking hours seeking and pursuing the very best help for this child and her family - irrespective of her byline. This child resents her, and I would bet that it's because she is narcissistic and therefore incapable of being an empathic parent. I'd be willing to bet significant money on this. Actually, I'd be willing to bet my life on this. Most parents would look first at themselves and their parenting if their young child was having major issues. To not look first at yourself and what you are potentially doing wrong is a very strong sign that you are a big part of the problem.
The inability to feel emotion, as it appears Lanza might have been experiencing, can be based in the lack of love and nurturing as a child, including the inability of a parent/parents to see and acknowledge their child as an individual, self-determining, separate life from their own. The lack of proper nurturing during critical periods can lead to a lack of conscience in the child. This is what leads to fourteen year olds in the inner city being capable of cold-blooded murder. In the end, the cumulative effect of all of this is tragedy - often involving those who were not connected in any way.