“The kid murdered his mother in order to gain access to the firearm.”
It has been reported that Lanza had access to the safe without having to kill his mother.
Wikipedia states “Investigators found Lanza was fascinated with mass shootings, such as the Columbine High School massacre, the Virginia Tech shooting and the Northern Illinois University 2008 shooting. Among the clippings found in his room, there was a story from The New York Times about a man who shot at schoolchildren in 1891. His computer contained two videos of gunshot suicides, movies that showed school shootings and two pictures of Lanza pointing guns at his own head.[173][174] It was also claimed that he had edited Wikipedia articles about mass murderers.[175]”
Even if his mother was not aware of his obsession with mass murders (and kept a scorecard) he likely gave some indications that he was. That would have been reason enough to change the combination on the gun safe. In any case all reports that were that he was a very strange child which would have justified additional precautions.
As I mentioned, it is certainly possible to use 20-20 hindsight to revisit the whole story about Lanza. I think it is logical to believe that his mother wasn't suicidal, and pretty much any law enforcement agency or even his psychiatrist or GP would have taken action if they knew his intent and plans.
At the time of the incident there was speculation that Lanza decoded the combination of the safe by trial and error. Some kinds of people with mental illness have the patience to repeat procedures many, many times more than is typical for people without that kind of illness.
So perhaps all firearms safes should require 8 or 10 digits instead of 4.
Blaming his first victim seems a bit heartless. I tend to blame the person who commits the crimes, and if you want to assign blame elsewhere assign it to the professionals who failed to properly treat or confine Lanza.