This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Singularity
The video game:
The Reverts and those sounds they make.
If you look closely, you'll see that some Zeks are made out of 2 people fused together, probably the same person in 2 different timelines.
You spend the entire game reverting tapes to hear messages. One of those tapes, for no apparent reason, suddenly causes a very short cutscene with a screaming corpse in your face. You'll hesitate to approach the next couple of tapes afterward.
After the first time you go underwater in the game, you emerge and... are greeted by at least two dozen horribly mangled corpses of Russian soldiers, all hanging or impaled. Although on a smaller scales, scenes like this happen time and again over the course of the game; the Zeks seems intelligent enough to keep reserves of food.
You can often find corpses of people who died when the Singularity exploded or in the aftermath, when the mutants started to swarm Katorga-12, often with notes or audiologs nearby, and you can only imagine how horr ible their deaths were. An early time echo shows the teacher of a female class telling her pupils to take refuge under their desks - back to 2010, and their little corpses are all there. Much later in the game, you can swim inside a cistern, towards a grating with a large cache of E99... and find dozens of corpses of poor souls who drowned underneath.
The "kill them both" ending. It involves Katorga-12 detonating with enough force that the graphic shows a crater reaching from China to Alaska, with the entirety of Japan simply vanished. That's about a 3000 mile diameter. The asteroid that is believed to have wiped out the dinosaurs, and most of the rest of life on the planet? Left a crater of a "mere" 110 miles across. Honestly, the Zombie Apocalypse that followed seems a bit redundant.