Instead it can come from the reaction of the living humans involved, and how they respond to the state of fear and violent chaos brought about by the zombies. While Horror is assumed to be an inherent part of the zombie apocalypse, not all the horror and conflict comes from the zombies themselves.
As society collapses, it's up to the Big Damn Heroes to fight their way to safety or keep shooting until things blow over. With each victim they claim, their numbers swell, and no force on Earth can contain them. Whatever the cause, the result is the same the recently dead have risen, en masse, to feed on the living. Maybe an alien space probe broadcast a weird signal at the Earth, or fell to Earth and brought radiation with it. Maybe the Synthetic Plague the government was working on got unleashed. Within the past couple days or hours, something very strange has happened.
