He collected from them a thick description of their thoughts, feelings, intentions, etc., when committing violent acts, and of their pasts. Then he sat down and compared their accounts to see what irreducible minimum they all had in common. He found violentization. I discuss the methodology of Athens's work in my book. Athens didn't find animal abuse in all his violent subjects, so it's optional, but obviously it's a way to rehearse violent performances.
Brutalization is the typical experience of a small child, with warning signs that are obvious.
Athens found boys moved from brutalization to belligerency typically by 9-11 yrs of age. I've looked at much of it, and it doesn't prove anything. It consists of statistical correlations of dubious connection to violence. In those days, almost everyone brutalized their children, to prepare them for a very violent world. And of course there were no mass media.