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Nightmare Fuel: Resident Evil Gun Survivor
In the first Survivor, you learn from a file that Sheena Island, the mass production plant for "Mr. X"-style Tyrants, has achieved this role by making use of a hormone found inside of teenagers' brains, and it's produced when they're frightened/stressed. Umbrella knows maximum fright/stress levels are caused by excruciating pain. Yes, Umbrella tortures kids to death in order to further their goals. Just let that sink in.
Worse, they open their skulls while ALIVE AND FULLY CONSCIOUS in order to get said hormones.
Vincent Goldman himself is a creepy, creepy monster far worse than any of the zombies in the game. He's so bad that the people under him were trying to get him ousted from Umbrella for the sick stuff he was doing by reporting to corporate headquarters, and the game indicates Umbrellawould have done it (or at the very least, Vincent took the threat of that happening seriously). This is the same group of people who gave Alfred Ashford his own private stock of prisoners to torture and execute and gave the position of Raccoon City Police Chief to a psychotic rapist.
His diary is even worse, as it reveals that he released the T-Virus on the island 'on purpose' just as an "Screw You" to the people on the island who wanted him gone.
The Trashsweepers. Utilizing Uncanny Valley to the extreme, these beings look like Gas Mask Mooks... until they start moving in ways no normal person should move ever. And the hooting...
The Hypnos Tyrant. It's first form is uncannily human, and is introduced by killing Vincent/Andy/the UT Commander. Its second and third forms are the scariest, however: with an inhuman grimace, monkey-like sounds, and the fact that it's always angry make it a terror to fight. Oh, and it took two missiles from a helicopter to destroy the thing.