How could a psychiatric nurse's son become a rapist/killer?

I watched a CBS 48 Hours Mystery segment "the Amber Dubois story." Amber was raped and killed by a CA registered sex offender John Gardner. In the segment, Amber's mother confronts John Gardner's mother who they said is a psychiatric nurse. Gardner's mother refused to speak with Amber's mother. Why didn't Gardner's psychiatric nurse mother have seen the signs in her son that could have prevented this tragedy from occurring? (Gardner also raped and killed Chelsea King.)