Tree's entire scenario. She's forced to relive the same day over and over again, being killed in increasingly horrible ways, and no matter what she tries to do or where she goes, the killer finds her. Is it any wonder she has a breakdown after seemingly killing the Baby Face killer in the last loop? And you can't really blame Tree for looking terrified when, in the final scene, Carter tricks her by making her think she's still stuck in the loop.
Most of the attacks by the masked killer, except when they cross the line twice.
The scene where Tree is rooting around in Gregory's office and comes across a baby mask, raising suspicion that he himself might be the killer. It makes the scene immediately after, in which he is attacked by the actual killer, all the more effective.
The killer blowing up the police car with her inside it. This one is made extra creepy with the police car's light flashing blue on the mask and Tree's pounding on the window for them to tell her who they are and why they're doing this.
The Hope Spot where it looks like the killer's been defeated, only for the day to reset itself once more.
While the goofy baby mask may look like Nightmare Retardant to some, others may still find it scary when it's worn by a killer.
The way the killer is just so determined to kill Tree, no matter what it takes. The killer may not be in a loop, but originally they merely planned to get her at her birthday party. When she stops turning up for that, they follow her no matter where she goes.
