Horror?

In response to Ellen's blog, I agree with a final point she made.
When we were discussing the film in class and how the final scene related to the traditional horror movie, with the rain and the crawling through the mud and the sense of helplessness to escape the "monster," I didn't find that frightening.
I think the most psychologically disturbing part was that one of the children was crawling along with a knife as well.
It is rather scary that these adults decided to take matters into their own hands and mutilate another human being, yes.
But what's even scarier is that they had a child help them do it.
In addition, Ellen raises a good point about consent and understanding.
If this individual has a mental handicap, how do we know we have their consent to display them like this?
Also, how do we know they fully understand that this is just acting, it's not real, and it's not okay to run around with knives?
I'm going to assume the filmmakers took the time to explain this to their actors, but still, it makes you think.