Horrifying

I just watched a film called "The Girl Next Door" that was based on true events.  It was about two girls whose parents died in the 50's. They go to live with their Aunt who is sadistic.  She assults them both, torturing one of them indescribably!  It had me in tears, knowing that this actually happened (or at least most of it.)  It reminded me of the book "A Child Called It" that I read on a train once while backpacking Europe.  In that case, it was the child's own mother who abused him.  How is it possible for people to be so sick and twisted?  It's bad enough that people come across strangers on a daily basis who are psychotic, but to think that some people find it within their own homes is so quite horrifying!