This movie absolutely destroyed me.
To be fair, I am very affected by the sadness and trauma of others, so it's not surprising that this movie almost killed me. To borrow from a comment I made on another user's post "This movie was a 2 hour long gut punch, and the end was a fever dream." It was so very traumatic, exhausting, uncomfortable, and TERRIFYING. And traditional horror movies do not ever scare me.
My overwhelming feeling for most of the movie was profound sadness. This family torn apart, the horrible things they say and think... the panic attack that Peter has and when he asks his friend to hold his hand? That was one of the times I actually cried. His numb stupor after his sisters head gets knocked off by a telephone pole(!!!!!). His mother's screams when she finds her headless daughter in the back of the car. The desperation when Steve splashes Annie in the face with water. The two times (one reality, one dream) Annie says just awful things to Peter. Peter smashing his face into the desk. Peter screaming/pleading "Mommy!" as Annie tries to get into the attic after him. These are all times I felt overwhelming sadness. Tons of other feelings: anger, disgust, terror, etc. But huge amounts of sadness that I've never felt during other horror movies.
Let me preface this by saying I know what the director has said about his vision and "what the movie really means." But I've never cared about a movie enough to actually fundamentally disagree with the person who created it before. self-deprecating eyeroll
This movie as a straightforward demon-possession/ occult movie does nothing for me. The whole time I had no doubt that it was a family torn apart by mental illness and that devastated and terrified me.
I'm going to post my inexpert interpretation as a comment. It won't be a synopsis, but there will be oodles of spoilers.
*Edit: I thought the movie was great. I don't know if I'll ever see it again.