Nothing would have stopped Lanza. He was severely mentally disturbed. You cannot prevent the irrational by using simply outlawing it. If we could prevent irrational behavior by out lawing the behavior, this would be a perfect world.
It is interesting to note that the media didn’t really delve into the background of the Arizona shooter or the Colorado movie theater shooter very deeply. If they had, they would have found that both of those shooters were deeply steeped in leftist political ideology, that may have exacerbated their mental conditions. The Arizona shooter was a graduate of a William Ayres designed Small School in Tucson, an experimental high school which emphasized political activism and organization over test scores. The Colorado shooter was a PHD candidate in neuroscience, which is the study of the use of buzz words and phrases as emotional triggers and how these triggers can be used for political persuasion. In other words, mass brain washing.
May be it was the far left ideology that caused those two shooters to go over the edge, the hate and the emphasis on the State as the only support system, no religion and no God.
The media never delves into anything that may contradict their pre-determined narrative.