For me the Book background is not so much on the reaction of parents, but rather on the way in which the attitude of parents in the past, the way it students have their child, and more generally their vision of the world, could make their child what it is.
From the beginning of the book we feel that teens have committed an atrocious act, so that one has the feeling that parents are people "like us", especially the narrator and his wife, auquels can identify himself by browsing First pages. We then said "violence is born of nowhere! How horrible! It can happen to everybody" and then we gradually understand what lies behind these "ordinary" people, "normal" and that is c ca, precisely, not "nowhere" that is born this abject violence.