Here is a doctor's opinion on the matter: http://www.slate.com/articles/health_and_science/medical_examiner/2012/12/mental_illness_in_the_connecticut_school_shootings_we_may_never_have_a_diagnosis.html
Who will ever know? It is possible that he killed his mother in a fit of rage and that everything went downhill from this moment when he singlehandedly destroyed his whole life. One does not even have to have a mental illness to act like a monster and then implode.
What is thoroughly disquieting about the recent mass shooting, apart from it, is the reaction of the public toward people suffering from mental illness. It is as if, for want of another scapegoat such as people of other races or people of other religions, one turned against mentally ill people...