The plot of this movie confused me a little.
When she was a human, why did the Slit-Mouthed Woman attack her kids, comforted them, then attack them again?
I know that she's sick, but what caused her to be like this?
"Aim at my neck."
She tells the teacher that.. so where does "Am I pretty?" come from? She doesn't even wait for a response.
What I got from the movie was that the women who mistreated their children were always women who were sick with some unknown disease - they are always coughing, they have to cover their mouths so as not to pass it onto somebody else, - or may be there's like a spirit who goes from one lady to the other killing children and finding another host as soon as its present host gets killed.
Aiming at their necks doesn't really work. The women die, but the spirit move onto the next host, invading her.
And the "Am I pretty?" thing may come from the fact that she's disfigured and has to hide her face. She knows she's not pretty with a scar like that, so that's why she doesn't wait for the answer - she already knows that. It isn't explained in the film, but it would make sense, wouldn't it?