The whole movie has an ominous, almost suffocating atmosphere. It's a while before anything terrible truly happens, and yet every frame is nearly oozing with dread.
Every time Kat starts stabbing someone, she just won't stop. She just keeps stabbing over and over, long after whatever damage she's inflicted is sure to be fatal.
The Foreshadowing scene where Rose pulls out a drawer filled with long sharp kitchen knives. What makes this scene so unsettling is not just the murder spree that Kat goes on within minutes, but the realization that items for daily use can be used by anyone, anytime, anywhere for cruelty and murder at a moment's notice and in the right circumstances.
The scene where Rose goes down to the cellar and sees Kat worshiping the demon in the furnace. The first time we see the movie, it's not entirely clear what is happening, but it's the first serious clue that something is terribly wrong with Kat, besides her just being kind of odd.
Kat in general is a deeply off-putting person. Even before we find out the nature and severity of her issues, her overall demeanor is downright creepy. Even when viciously murdering people, she does so in a detached, expressionless way.
The demon. We only ever see it cloaked in shadow, making it look quite a bit like the Grim Reaper.
When the police officer confronts Kat in the furnace, she is standing there with the severed heads of Rose and the two nuns as an offering to the Devil. As she faces the officer, she proclaims, in a voice that isn’t quite hers anymore “Hail Satan.”
Kat's face becomes a Nightmare Face after she's possessed. It begins the morning after Rose saw her boyfriend, when Kat makes a call on a payphone. The Nightmare Face isn't obvious yet, but hints of it are there when she has her "you smell pretty" conversation with Rose. It becomes more pronounced when Kat vomits at breakfast, and speaks in a Country Matters way towards Ms. Drake. It gets even worse after she has killed Ms. Drake and Ms. Prescott. It truly becomes the stuff of nightmares after Kat murders Rose.
Before Kat herself becomes Nightmare Fuel, the situation she finds herself in is quite anxiety-inducing. She has a vivid nightmare about her parents seemingly dying in a car crash; the next day, they are no-shows to pick her up for winter break, and don't even answer her calls. Everyone around her seems to just offer cheap platitudes, doing nothing to ease her sense of dread. Rose, her only companion left, leaves her alone in the dorms late at night. Then the phone rings...
The ending is this and a Tearjerker from Kat’s perspective. She has no family, no friends and is on the run from the law. She has murdered several people in order to get possessed again but it doesn’t work, so she has to live with the knowledge that, without demonic influence, she’s a murderer too. And considering that demons exist, she’s most likely going to Hell after she dies because of the horrible things she’s done unless she somehow lives long enough (without getting arrested) to pull of some serious deeds of altruism. She’s completely alone, has nowhere to go, and her soul is likely damned. That would be a nightmare scenario for just about anyone.
