This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Suffering
Some of the earliest monsters encountered are the limbless corpses of stabbing victims who have had their limbs replaced with very long blades. They would approach you by clinging to the ceiling and then dropping on top of you. The only warning one would have would be the sound of metal scraping across cement.
Hulking zombies with guns sticking out of their back, corpulent corpses that spew exploding rats, cute little girls who turn into cackling, flaming midgets... The Suffering is not a Third-Person Shooter for the faint of heart.
In The Suffering, there's an area where you see three open, empty prison cells. If you walk in them, you can hear the past prisoner in there say something, one about Dr. Killjoy, one about how he raped a girl, and the final one just being a deep, raspy voice saying "I found a way out." Most of the voices are creepy as hell. Case in point: Walking to close to a hidden ancient, decayed, meaty corpse outside the Asylum will result in a lengthy voice clip of a woman screaming Dr. Killjoy's name over and over again. In delight.
"That little girl wasn't so... innocent by the time I was done with her..."
Let's talk about the Asylum a little more, shall we? It's the one with almost no enemies, just you wandering around in the dark. There are a couple monsters in the basement, there's one fellow who's hiding out in a side room trying not to get killed, and in an ordinary room at the end of a hall, there's a man so horribly mutilated that in real life he'd probably be dead, flopping around with no arms and legs and trying to talk while choking on blood. You actually gain points on the Karma Meter for putting him out of his misery.
Hermes didn't seem extraordinary frightening until you found him hovering over Horace Gage's corpse, saying, "I always looked at their bodies when I was done."
The fate of the unlucky CO that wakes up inside Hermes' gas chamber, largely because it is very easy to imagine actually experiencing. What made this worse is that there is no way to save him.At all.
The dialogue in Hermes' boss fight. "Once you've killed a man who was begging for his life, and everyone congratulates you on a job well done, nothing can compare." All in that uniquely terrifying voice. The dev team must have really liked Hermes, because while his boss fight only lasts about his minute if you know what to do, his voice clips can go for quite a while longer than that without looping.
Seeing Horace's electrocuted ghost approaching you from behind through a security monitor. This bears repeating: You're looking through security monitors innocently, looking at all the empty rooms, until you cycle to the room Torque is in and OHGODWHATTHEFUCKISTHAT.
The Nooseman in Suffering 1. All you see is a puddle of blood on the ceiling. But if you get close from the puddle the Nooseman appears. He is an upper bodyflayed and hung. He proceeds to strangle you before climbing back on the rope he was hung with. Doubling the creepy factor is that these things do not obey the laws of physics, period. They're literally described as "ripping themselves from the ceiling." Not falling, ripping. And with all enemies in this game, the Nooseman represents some form of violent death. Of course there are those who were hung, but in particular, they represent those guards who were cut in half, flayed and hung by rioting inmates after a mine collapse buried several of them alive. Of course there are enemies that represent those who were buried alive...
Something that's also frightening is Hargrave. Not because he's any kind of biblical monster. He's literally gone mad with power and is starting to systematically kill every inmate he comes across.
The bad ending. Just... the bad ending. You thought you could get away with mass murder? Nope. You're forced to face your own angry family and the ugly manifestation of your evil. And just after you figure out just how you murdered your family, you lose control of yourself while some unfortunate sailor is trying to save you, being crushed by a boulder in the process. Oh, and the family picture? It is tainted by blood.
This is us, T. Us without you, the only way we can possibly be happy. Goodby forever - Carmen
The Suffering 2
The Suffering 2 equals and quite possibly one-ups the original in terms of sheer mind screwery. Let's recap, shall we?
The Gorger Monster born from a priest feeding human meat at a soup kitchen
The Arsonist Two people fused together by fire!
The Creeper A pimp who uses his dead prostitutes as weapons!
The monsters that were buried alive get a bit scarier in the second game's descriptions. Jordan observes that anything with a spine that can bend that way must be in constant pain, even if it's unable to die.
After killing some waves of goons near the end you learn they sacrificed themselves to your bullets just to screw with your mind. Humans. Not monsters.