This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Zero Day
The footage captured by the high school security cameras.
The entire sequence where Andre talks about the guns in his home. "54 seconds is all I need to be able to hide this sucker and come after you."
The scene where Andre and Cal are making pipe bombs and wondering what kind of injuries they'll produce.
The very premise of the movie. Two normal, teenage boys decide to go on a shooting spree in their local high school, killing without any mercy. The worst part is how Truth in Televisionit is.
The creepy part isn't the usual media-based mentality that the shooters were unstable or violent: Cal and Andrew are shown to be normal and decent kids, who decided to do a school shooting because they wanted to. That makes the entire premise more creepy.