More info. The killer had bought many guns, and so had his mother, who was something of a gun nut herself. Multiple ARs and AKs, pistols, etc.
He had been admitted to a psych hospital, was intermittently on psych meds, sometimes wouldn't take them, the mother knew all of this - they lived together - and she was a nurse and well informed on mental illness issues.
She seems to have been somewhat in denial, told people her son was doing great, in college, etc. In reality he was a loner who was apparently seldom at school and was a gun nut.
http://mobile.nytimes.com/2015/10/06/us/mother-of-oregon-gunman-wrote-of-keeping-firearms.html?_r=0&referer=http://news.google.com/news/m/more?ncl=d7WxxCg7QajH4BMn_dpJagEzd2TiM&authuser=0&ned=us&topic=n