Resident Evil 0 doesn't have the REmake's Crimson Heads, but it makes up for that with a new enemy called a Humanoid Leech (often referred to as "leech zombies"), a swarm of T-Virus leeches that gather together into a human-like form. Aside from being full-blown Demonic Spiders, the things just love to hang around in narrow, claustrophobic hallways, move with a disturbing, boneless walk, and have a really disturbing musical theme. What's more, humanoid leeches can and will make their way into rooms that were previously empty, meaning that you will reenter a "safe" area, only to find as soon as the screen finishes loading that one of these abominations is right in front of you. Oh and by the way, you absolutely must destroy them with either Molotov Cocktails or Flame Rounds from a grenade launcher (or the Handcannon if you have it on another playthrough). Otherwise, you'll render it without a top and then you have all of five seconds to get as far away from it as possible as it swells up to a bulbous mass and then explodes launching leeches all over the place. This explosion can kill your characters (especially Rebecca) if they're too close.
The worst part? They look like normal people. The first one you find sits there looking like an old man before his head FALLS ONTO THE FLOOR.
And further dwelling on the scene from above the old man's body then splits into chunks as Rebecca screams in horror and then we're treated to the chunks all revealing to be leeches that all begin to huddle together in a grotesque and slimy mass and till they take on the shape of the man again only this time with obviously inhuman skin tone and then as the face reels forward, the camera zooms in as the thing stars at the screen with creepy, white, soulless eyes.
Becomes even more unsettling on a second play through when you come to recognize that the old man the leeches took form of is James Marcus.
In the Power Plant level, in the same room where you rescue Billy Coen after he took a tumble into the water, you find a stack of skeletons all piled on top of one another with grime on them. These were all of Dr. Marcus's victims after he went mad with testing the T-Virus on humans.
More of an instance of Fridge Horror combined with Tear Jerker, but Rebecca Chambers' last meeting with Enrico Marini, her squad captain becomes a whole lot chilling considering he's murdered only a few hours later by Albert Wesker, his commander and co-patriot whom intentionally sent everybody on this wild goose chase to see how they'd fare against B.O.W.s Her words really hammer in the chilling realization:
Rebecca: "I never saw him again."
Billy, after parting ways with Rebecca, disappears into Raccoon Forest, which is still infested with the zombie dogs. To this day, Capcom hasn't explained what happened to him.
In the kitchen on the back of the train, there is a giant fridge which you can't open. So just walk past it and ignore it. Once you enter the next room and back, a zombie bursts out of the fridge the second you walk past it. A very good (yet likely very predictable if you're familiar with the series) jump scare. Also in the fridge was a bottle of booze, hinting what that person was doing during his last hours hiding from the zombie outbreak.
There's a gas chamber in Marcus's main lab. And it's not even hidden or disguised as something else; it's right out in clear view for people to see. And there's a body in there that becomes a zombie once you remove the gas from the chamber. Clearly, Marcus wasn't below sparing anyone (let alone his colleagues) a terrible death for simply knowing what he was doing.
Most of the files you find in the Research Center are chilling reads as most of them are from attendants of the center and either witnesses or victims of Marcus's cruel research. Most of them are written with the writer fully aware Marcus was on to them.
James Marcus's descent into madness which you can read about slowly through many files littered throughout the game. The man started out as a typical scientist only curious by the virus (in fact, he is annoyed when OswellE. Spencer constantly approaches him about heading the training facility). He concedes eventually, and of course, with too much power in his hands and his unhealthy obsession with the leeches, he slowly becomes a monster.
Marcus also didn't see his leeches as test subjects or even pets, but rather his children.
The basement of the training facility is all kinds of disturbing. Not only is the theme downright creepy , but Rebecca eventually stumbles upon to her horror a full torture/execution chamber complete with whips, electric chairs and even some iron maidens which she examines to have a lot of dry blood on them.
Further down in the basement you discover some sort of battle chamber/maze pit that at the push of a button from the command chamber overseeing the complex adjacent in the other room locks the pit with barred doors and drops Hunters in for some sort of battle sequence. The only skeletal remains you find in this pit are of people. Knowing Marcus, the implications here are a lot worse than you think.
The Hunters' theme is bound to get you running to create some space between you and the creatures.
The Proto-Tyrant (the prototype of the Tyrant from the first game) is the very epitome of body horror. Its lips have deteriorated, there are gaping holes at the sides of its mouth, it violently twitches like it's possessed every ten seconds or so, and to top it all off, its spinal cord is out in complete view. It also came insanely close to cutting Rebecca's last route of escape from the facility with a flick of its claw had she not already have called for the elevator.
The fact that even when Marcus reveals how Umbrella ultimately backstabbed him and had his two favorite students assassinate him personally you simply cannotfeel anysympathy for him after everything you've learned about him throughout the game. Seriously, when you make the likes of AlbertWesker and WilliamBirkin look like saints in comparison, you really oughta rethink your life. Not to mention Wesker and Birkin tossing Marcus's corpse into the Sewage Plant comes across like poetic justice considering this is exactly what Marcus did to the bodies of everyone he did away with after experimenting on them.