Important detail to understand the ending - SPOILERS INSIDE

Close to the end, when Sarah wakes up there is a very important piece that was not made clear and definitely change everything ...


... on the bed there is someone with the head covered with a pillow. We assumed it was Mia. The scene is only shown for a very short time. But if you check check carefully it is an adult and not a kid. And it seems to be a male adult. So I now think that when the husband came she killed him too.

I think there is nothing supernatural in this movie, it is just Sarah getting crazy due to remorse.