The most chilling thing was a passing comment (spoiler)

When they have the mother tied up, and she is confessing her part in allowing her son to be abused by the father, she mentions one small detail - and I found it really disturbing. I think it's easy to just go past that, like it's not much.

She says something about how she would give her son an enema and a shower before bed. This comment takes the abstract idea that the father was sexually abusing the son, and makes it very concrete and clear. That she would make sure her son was clean for this act, and that her husband can satisfy himself, and then come to bed to be affectionate with her.

For some reason, her speaking this detail was really jarring and sad.