I need help creating a monster for my novel?

I think the scariest monsters are ones that look weak or innocuous but have the ability to kill very fast. We expect a big hulk to tear us to pieces, it's not scary, it's cliche, but a little girl who can transform to a creature that kills us or makes us kill ourselves with psychological torment, now that makes me wet my knickers. As far as naming: She looked like a kid, that devious grin was an invitation as much as it caused my neck hair to bristle, I didn't know her name, but the villagers called her the Wicked One.