[Blarg]

Don't go for the "parents seem nice and normal" thing.

Remember the unabomber? His brother, the LESS socially maladapted one, lived in a ditch for a couple of years. Parents didn't seem that abnormal.

In the book Whoever Fights Monsters, by Robert Ressler, the guy who started up the FBI VICAP(Vicious Criminal Apprehension Program) and coined the phrase, "serial killer," wrote about Ed Kemper, whose mom was a quite normal lady, seemingly. Kemper wound up killing her. Ressler, describing Kemper's upbringing, said his mother made her house virtually a "factory for producing serial killers", to paraphrase.

Just like the story of a couple, the story of a family is one that outsiders can never really know, though it might seem perfectly apparent. Most people know not to air out their dirty laundry in public, but there is plenty of dirty laundry about. Putting on a front is an elementary survival skill in life even for harmless people with quite ordinary lives. That we can walk by people's strange and varied lives unsuspecting is no surprise. The world would find ways to punish them severely if we couldn't. All anyone is necessarily doing by putting up a socially acceptable front is following the path of least resistance in a landscape of stark alternatives.