Nancy Lanza, the mother of the shooter, knew that her son has Asperger’s syndrome. She knew that he had problems understanding and relating to other people.
Nevertheless, she kept in her home not just a semiautomatic rifle but two semiautomatic pistols. Even worse, she took her son on trips to teach him to be a shooter. Worse than this, she obviously kept her dangerous weapons in places where her son had easy access to them. She paid for her ignorance in the worst possible way. But even worse, she was responsible for the murder of 26 other people.