Cinema, and particularly horror cinema, is obsessed with evil children - there are so many examples so I'll just mention a few favourites: Damien (The Omen films), or the Children of the Corn, or Sadako (Ring). Generally the evil is explained or justified in supernatural terms, and there's no attempt to portray these children as having human motivation. Another, different breed of evil child is portrayed in Lionel Shriver's novel We Need To Talk About Kevin, describing the lead-up to a Columbine-like mass killing. Kevin is the opposite of Damien - utterly human and so when we find we can't answer those fundamental questions about the nature of his evil, we can't dismiss them as demonic either.
Esther is every bit as creepy as Damien, but her physical brutality is matched by psychological rather than paranormal abilities. However Esther's motivation isn't explored, and both this and any conclusions about the nature of childhood evil are sidestepped by one of the film's two major twists (the other, a brilliant visual twist, concerns the real meaning of the paintings) so she remains a cipher for pure evil rather than a real character. To this extent her adoptive, deaf sister Max, played by Aryanne Engineer, is much more of an interesting character - she walks a tightrope between complicity and resistance, motivated by fascination with and fear of Esther and a sense that no-one else will be able to protect her.