The kid murdered his mother in order to gain access to the firearm. The kid's father was no longer married to or involved with the kid.
It is certainly possible to debate the wisdom of every aspect of the care provided to the shooter, including having a firearm present that he could murder someone to obtain, but there is either blame all along the path of the shooter's life, or he is responsible for his actions.
Blaming the parents, including a father who wasn't even there is unjustified.
“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.