Serial Killer X - Condemned
It's clear that somebody has issues when they decide to start killing other serial killers. Not only this, SKX (as he's affectionately known) kills his victims in the same sick and twisted fashion that they killed their own. With a strange obsession with the main character Ethan Thomas, SKX uses mind games to manipulate the poor detective, which provides Condemned with some of the scariest set-pieces in video game history. SKX isn't just a psycho; he's a criminal mastermind with the kind of power to send a whole town (and in turn the player) into the depths of chaos.
Pigsy - Manhunt
Rockstar's gruesome Manhunt is full to the brim with the mentally unstable. In a twisted gameshow orchestrated by an even more twisted director, the dregs of the criminal underworld are all thrown into a sick survival match with one James Earl Cash as the star. In this frightening gameshow, one enemy was slightly more unnerving than the rest; Pigsy - A chainsaw wielding man-pig that relentlessly chased the player around a decrepit building. More horrific than this was the accompaniment of disturbing snorts that Pigsy made as he ran. This list contains a fair few head-cases, but out of all of them, Pigsy is the one you'd least like to meet in a dark alley...