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Nightmare Fuel: Dead Rising 3
Albert Contiello, the organ-harvesting doctor representing Greed, may be the darkest psychopath ever fought in Dead Rising. First of all, you have to fight him while under the influence of some drug injected into you, which makes you hallucinate other survivors as clones of Albert, so you'll have very little clue if you're attacking the real thing or an innocent person (and considering the way the other survivors are behaving, they're probably going through the same experience). You'll also occasionally hear screams of terror as Albert tortures his other victims by cutting them with his surgical saw. What's worse is that every once in a while you'll black out from the effects of the drug, but not before seeing Albert and his "clones" twitch about in an eerie fashion and laugh maniacally. Even his death scene is more disturbing than average for the series, as he cuts himself open with his own saw and pulls his own intestines out while hallucinating that he's being eaten by zombies.
Darlene Fleischermacher, the psychotic competitive eater representing Gluttony, is a disturbing mix of both this and Nausea Fuel. It's made even worse as she violently stabs a starving guy to death just for trying to get some food. Her death scene, falling over and choking on her own black, bloody vomit, is also right up there with Albert's as one of the most disturbing seen in the series.
Darlene's Nightmare Face when she screams before her battle also counts:
Dylan Fuentes, the insane fetishist representing Lust, is pretty damn creepy. The guy keeps innocents trapped in the back room of a porn store, wears a semi-concealing BDSM mask and has noticeably sharp teeth.
Though the boss fight against him is a mostly bombastic martial arts reference, "Zhi" Wong - the psychopath of Wrath - is fairly terrifying, particularly what he had been doing when Nick finds him. When Nick first arrives at his garden Zhi welcomes him, leading Nick to believes that the place is a safehouse/sanctuary... and then he sees the mutilated bodies strewn about the place. At first, Nick believes something so horrible could've only been done by zombies, but then Zhi calmly and politely explains that he slaughtered them out of anger. Nick is rightly horrified, but Zhi doesn't seem to mind until a zombie strikes a gong and disturbs his tranquility - at which point he goes into a homicidal rage and tries to kill everybody. Just think how many of those dead bodies lying on the ground were innocent survivors looking for shelter, thinking the garden was a safe haven as Nick did and welcomed in by a warrior who seemed to be protecting them, not knowing that at the slightest irritation their "protector" would descend upon them indiscriminately and cut them to ribbons...
Nick encounters a zombie whose head has become horribly disfigured into a fully functioning bee hive. This is revisited later when he comes across the mutated King Zombies that are walking bee hives and can fire bees from their mouths.
Sgt. Hilde Schmittendorf establishes her role in the game when she orders a guard to shoot a pleading man's wife dead. And then, without a shred of remorse, kills the man by snapping his neck. All with a disturbing sexual pleasure.
Diego's demise. He suffers a painful death from being hit with Marian's laser, which causes larva and queens to burst out of his eyes and mouth.
The president being turned by being force-fed a parasite. One second she's coughing up blood and the next she's completely and totally zombified.