This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Resident Evil Outbreak
In the FMV intro, a mortally-wounded soldier is eaten alive by sewer rats.
The Hive might be pretty awful for people that aren't too excited about hospitals. Especially considering that a nigh-invincible Leech Man chases you, and your partners turn into Leech Men when they die.
The cutscene on Main Street in Outbreak. The sheer scale of the outbreak hits home right then, and it's an Oh, Crap! moment both in-game and out.
"Wild Things", which takes place in a zoo full of zombified animals, such as lions, hyenas, and crocodiles. But the most terrifying is easily Oscar the elephant, or as he's now known, "Titan". As if ordinary rampaging elephants weren't scary enough, Titan is a rampaging elephant with decaying skin, jagged, bladelike tusks, and his entrails hanging out of his side.
The opening cutscene for the level. Ever wonder what happens when a zoo full of cute animals gets caught in a zombie outbreak? Animal lovers, mash the button to skip the cutscene or look away, or you will be in for this combined with a Tear Jerker.
The artwork for Nyx makes it obvious the dev team was just outright going for the Eldritch Abomination look. Boy, did they pull it off. A monstrous jelly being with a single core, capable of absorbing you and any corpses around. Including a Tyrant. The kicker? Some of the guys are still alive, including the Tyrant, as Nyx tries to devour and absorb them.
And it can happen to your characters too. If Nyx swings his arm down and knock you flat on the ground in just the right position he can suction you into his back if you don't react quick enough and wiggle the analog stick, or have orange-caution health and below. From then you're helplessly trapped in Nyx's writhing flesh while your Virus Meter runs freely as if you got mortally wounded and crawling all while your teammates desperately try to kill it and save you (pretty difficult to do if you're carrying the big guns).
In Outbreak 2, an Abandoned Hospital in the middle of an empty woods, with zombies infested by plants that pop out from underneath your feet. And then, of course, there's the mutated, axe-wielding serial killer stalking you through the place intent on chopping up your corpse and feeding it to the monster plant in the basement that he believes is the reincarnation of his dead wife.
The Hive. The creepy abandoned hospital setting and the fact that the Leech Man would pop out of the air ducts. Constantly. And he was invincible. You spent half the time going through that level praying that you'd manage to get what you needed before he showed up in the room you were in... or that he didn't block the door. He also showed up more often when you were wounded and could attack you from 20 feet away. And if your partners go down he or she becomes a new Leech Man.
The best part? You can kill the Leech Man by luring it to a specific room. Planting a blood packet on the ground, waiting for him to show up, and watching as he laps it up, then crank up the heat and watch it writhe and die. Problem is, if your dead buddies become Leech Men, they're not going to fall for it again.
We get to see the middling stage between a Crimson Head and a Licker, "The Suspended". The mutations aren't pretty, as we can still see the woman the zombie was before it began. To make matters worse, incidental dialogue implies George knew her.
The Suspended has a hole in her abdomen and, on certain difficulty levels, some Lickers will try to protect her. Fridge Horror kicks in when you realize that a human woman may have somehow been turned into an incubator for Lickers, and turned partially into a Licker herself. The game itself does not seem to have any answers as to how she transformed, either accidentally or from Umbrella's usual machinations... You decide which is more disturbing.