Most difficult part to watch. *Spoilers*
It seems like everybody regards the fatal stabbing of Page and the rape of Mari to be the worst parts. I found both to be very difficult and sad to sit through, but to me that was not the worst part. The worst part for me was when the sh-t started to hit the fan between John/Emma and Francis. You could tell from the very beginning that when John and Emma discovered that the 3 were the ones that had done all those horrible things to their daughter, they were more disturbed, upset, and terrified as opposed to violently enraged.
It was more of an attempt to trap him and keep him from causing harm or endangering them and their daughter, but Francis is obviously a violent psychopath so they had no choice.
It was like the most violent form of self-defense. It wasn't out of malice, but fear. They hated doing it, but had to. When they had Francis' head shoved into the sink trying to deprive him of breath when he manages to pull out the drain...followed by his hand being shoved into the trash compactor with the blood quickly reddening the sink water followed by his scream of immense pain put knots in my stomach... Mind you, I hated this character and had no sympathy for him after he was killed, but during those moments I almost had to look away. I was thinking 'just kill him already, I can't take it!'. Quite impressive for a horror/thriller that shows the true ugliness of that kind of situation. Aaron Paul is a stand out actor and I don't think the scene would have been effective without him.