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Nightmare Fuel: Postal
The reason for going on a killing spree in the first place. The Postal Dude seems to think that the town he lives in has been contaminated with a "madness plague", turning the town's populace into psychopathic killers, and that he must cleanse the Earth of those who are infected. And trust me when I say that he takes on this mission with absolute glee.
The diary entries in between levels, e.g. "10/17/97 The Earth is hungry. It's heart throbs and demands cleansing. The Earth is also thirsty..."
The screams of the dying seem to qualify, as do the unsettling menu screens, very unsettling loading screens, and the total Body Horror of the credits screen.
Not so much the original game, but Apocalypse Weekend does introduce undead zombies into the mix, as well as many creepy encounters with zombified Gary Colemans (probably rendered more creepy after his death). A fanmade game, Eternal Damnation, included in the Fudge Pack edition of Postal 2, is full of Nightmare Fuel, especially in parts of the game where giant Body Worlds-like corpses chase you down the street.
In one house, you find that the bathroom floor, sink, and bathtub are covered in blood.