She was a little girl who was adopted along with her brother in the 80s. She started scaring her new parents by being really violent and threatening to kill people. They did some digging, and it turns out she was sexually abused from age 19 months and now could feel no empathy. She even tried to kill her brother by hitting his head against concrete. She eventually recovered and is now a nurse. She even wrote a book that I think would be interesting to read.
Bone-chilling, this documentary is.