I should point out two of the best zombie moments ever. One, in Thief: The Dark Project, dead bodies will moan at you. If you back off, that's your one and only warning. In Left4Dead, you try walking by that dead body (anywhere) and it starts moving. L4D infected, even though they're technically alive, can be very, very still.
Speaking of Thief: The Dark Project, the haunts are pretty scary, seconded only by the Servants in Thief 2: The Metal Age. And the servants are non-combatants.
An especially creepy experience came in Jedi Outcast and I had been disarming opponents all the way through Bespin. Finally, the pathway just ended, and with time, I figured out what happened. A Reborn was supposed to attack me, but he's activated by all the previous opponents being dead. The devs at Raven hadn't considered that some of us might be Jedi-like and actually spare the myriad of goons we had encountered. So I had to go back and take a walk on the Dark Side hewing down unarmed foes as the Reaper himself.
In Requiem: Avenging Angel [http://en.wikipedia.org/wiki/Requiem:_Avenging_Angel], when you kill some human combatants, they don't die immediately, and they'll wail and sob like... well like a guy who's down and bleeding out unless you put them out of their misery. The ESRB has since pressured developers to actually tone down the humanity of enemies, especially as they respond to pain and injury. According to them, a person lying there screaming because you blew their leg off might be disturbing to some players. Seriously, it has made the difference between AO and M for more recent games.[footnote]This would actually make for an excellent deterrant of friendly fire on civilians, or any other inappropriate targets, if they collapsed and started screaming from their injuries the way that real people, including soldiers do when injured and dying on the battlefield.[/footnote]
238U.