I feel like the horror bit, and the plot holes some people speak of, are less important than the underlying point. If you defer to others in minor ways everyday (your partner, your children, your friends, etc) you will ultimately have no boundaries and everything could be taken from you, most importantly your sense of self. When the husband is crying in the car it is spelled out pretty clearly. I feel like the plot was less about the "evil" couple and more about the "good" couple's willingness to give in when pressured. It starts with the daughter's bunny at the beginning: the husband clearly doesn't want to look for it but he does; the husband wants to go on the trip but doesn't speak up when the wife says no; the wife is then convinced by friends at dinner to go on the trip; when pressured to eat pork as a vegetarian the wife acquiesce, etc, etc. The atmosphere and ending scenes are pretty uncomfortable and terrifying/horrifying respectively but the underlying theme is what stays with you. The images serve as visceral reminders that if you don't use that agency and free will we are all born with, then, well, you might die naked in a pit via stoning. Metaphorically or otherwise. As for the children, well, they clearly won't be speaking up for themselves.