A closer look

The adrenaline-pumping undead horde shooter almost lacked the one thing that makes it terrifying: zombies. Early prototypes of Left 4 Dead featured the survivors fighting off regular, non-infected humans who were just participating in a very violent riot. Valve realized pretty quickly that mindlessly mowing down thousands of angry citizens was profoundly disturbing and ethically problematic, so they slapped some zombie makeup on the models to make the slaughter acceptable.