It turns out that his mother was - as reported by the New York Times - active in online forums talking about 1) guns 2) raising a child with Asperger's.
Ms. Harper, who divorced her husband a decade ago, appears to have been by far the most significant figure in her son’s troubled life; neighbors say he rarely left their apartment. Unlike his father, who said on television that he had no idea Mr. Harper-Mercer cared so deeply about guns, his mother was well aware of his fascination. In fact, she shared it: In a series of online postings over a decade, Ms. Harper, a nurse, said she kept numerous firearms in her home and expressed pride in her knowledge about them, as well as in her son’s expertise on the subject.PhotoThe Gun lobby has blamed this killing, so far, on mental illness, autism, loneliness, absent fathers, lack of heroism from the victims, and surely many other things. I'm going to continue to focus on the guns.
She also opened up about her difficulties raising a son who used to bang his head against the wall, and said that both she and her son struggled with Asperger’s syndrome, an autism spectrum disorder. She tried to counsel others whose children faced similar problems. All the while, she expressed hope that her son could lead a successful life in finance or as a filmmaker.