Not since Damien Thorn have we had a child whose stare alone genuinely sent shivers down your spine, much less her actions... Just try looking into her eyes in the poster on the main page for more than 3 seconds... no sleep for you tonight.
Esther's lifeless-like expression upon overhearing Daniel and Max discussed Esther's murder on Sister Abigail. This is a big clue to explain what really happens on Esther: she's mentally unstable, and moreover, she's not really a child.
Esther's Death Glare while staring at her 'paintings' and switching the lights of her room on and off, without any explanation at all why she's doing that. Nothing Is Scarier as this point because Esther hasn't done many bad deeds yet so it's pretty disturbing because it makes us wonder if there's really something wrong with her. Later, you know what kind of painting she's actually staring at.