This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Battle Royale
In general, even the novel is filled with huge amounts of Gorn.
The fact that every student in Battle Royale's Japan lives in a world where they could get placed in The Program no matter how nice they are.
Since other students won The Program shows how many people were willing to slaughter their classmates.
Your best friend might kill you, or will you kill him/her?
The horrifying part is this happened in the movie, and if the audience doesn't pay attention, they miss it. Remember those two girls who pledged to be friends forever? They're announced dead at the same time, in the same clip, during one of the updates. They killed each other.
The constant fear that anyone in your class may turn on you is very creepy.
KazuoKiriyama finds killing his classmates the same as playing an instrument.
When you learn that the kids are only in 9th grade it makes it even creepier that the government forces them to kill one another.
A manga scene where Kiriyama casually pokes out the eye of the class' former PE teacher, grabs the hanging eyeball and squeezes it definitely takes the cake. The whole scene is exhibited with rather elaborate, graphic panels and Kiriyama's disturbingly calm remarks.
"It's like egg whites"
There is a particularly disturbing eye scream in Battle Royale II where one girl's eyes are shot out by a sniper and she flails about wildly screaming for her best friend, who is in a nearby naval boat screaming for her - then the blind girl's boat explodes.
There is a particularly disturbing scene in the manga involving Mitsuko and Yuichiro. Yuichiro is dying of a bullet wound, and Mitsuko, who is somewhat deranged at this point after killing Tadakatsu and has regressed to her fucked-up childhood, decides to make him better...by raping him. Yuichiro begs for her to stop, as she is hurting him, and Mitsuko responds by stabbing the poor little guy in the neck with her sickle, seeing his cries of pain as a betrayal. As he dies, she is shown as a giant, damaged version of the doll her father gave her, asking, "How do you like my eyes now?"
It's very unsettling to see how quickly classmates (and even friends) start killing each other. (And without even counting the likes of KazuoKiriyama or MitsukoSouma.)
In the movie, Kiriyama's eyes after he's blinded by Shinji's bomb.