This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Children of Men
The human race can never again reproduce, and so basically everyone is waiting around for the imminent end to all humanity. Violence is rampant, and rule of law and moral standards have decayed, to the extent that illegal immigrants are summarily executed within moments of arrest.
The "The World Has Collapsed - Only Britain Soldiers On" ad in the train
The scenes where Julian and Jasper are murdered were so chilling in their swiftness.
The second one wasn't swift... That would have been better.
The opening scene—an explosion inside of a shop that Theo was just in, followed by the sight of a woman with gruesome injuries staggering out—really sets the tone for the rest of the movie.
Gruesome injury doesn't begin to cut it - that woman came out holding her own ARM!
In the background of the shot, just before the explosion, a couple greets one another and embraces on the sidewalk just by the shop entrance, much closer than Theo is. One second later, both of them are knocked to the ground by the blast. Just the total and utter Mood Whiplash that they must have gone through, even more so than the audience.
The scene where the bus is pulling into the Bexhill Refugee Camp. The antics in the prison area are deliberately and viscerally reminiscent of the Abu Ghraib prisoner abuse incidents, with prisoners being sworn at, stripped down, beaten, threatened with attack dogs, and sexually humiliated by sadistic guards running riot while The Libertines' 'Arbeit Macht Frei' plays in the background. To those who are aware of the full extent of the abuse at Abu Ghraib and Camp X-ray, the fate that awaits Miriam is all the more horrifying.