The most fun enemy to kill in a video game is a person. Robots are boring – they just explode into an unsatisfying shower of sparks and metal. Monsters and aliens are better, they come apart and they're bloody and gooey, but they don't have the same context as an actual human. There's something grotesque about shooting someone in a game. When your bullet hits them in the arm, or head, or chest, you feel it much more strongly because these are body parts that you own as well. It's spectacular, but it makes you wince. You can empathise with the pain, the impact, and it makes what you just did feel that little more tangible.
There's a moral element too - over robots, monsters and aliens, shooting people in a game is always going to feel ickier. Games have done a consistent job of reducing violence against people down to an empty, blithe test of reflexes. But that move towards fantasy, designed to remove or at least dilute the ethical dilemma of enjoying simulated violence, paradoxically makes simulated violence feel even worse. There's no urgency to it, no danger. You're just indifferently killing person after person, and neither you nor the game seem to care.
It's without splendour. There's no pain to it, because they act and are treated like cardboard pop-ups, but there's no unbridled joy either, since you're implicitly aware that what you're doing is grubby. This is why games like Zombi, Dying Light, DayZ, Dead Rising, Dead State, H1Z1 and Dead Island have grown so popular. This is why Call of Duty and Red Dead Redemption have additional zombie modes. In the zombie, you get an enemy who looks like a person, bleeds like a person and comes apart like a person, but doesn't have the moral baggage of a person.
There's also a built-in excuse to swarm the player with enemy after enemy. When, in Call of Duty, Battlefield or games like it, you're killing soldier after soldier, person after person, it feels mucky – it feels like the game is tacitly turning people into mere ludic challenges, into the moles of Whack-A-Mole. But zombies, despite looking like, even dressing like people, they move in herds.
The classic image is of zombie hands crashing through the windows, while more stagger in through the doorways. Zombies swarm, and since the nature of a zombie story implies that a lot of people have become zombies, and that the player is one of the few humans left alive, it makes sense that there a lot of enemies to fight.
The pop-up, gun range structure of shooting games - morally troublesome when it's people being reduced to targets, works for zombies. It's what zombies do. It's how zombies work. Shooters take representations of people and gives them animalistic, insect behaviours. The enemies in war games rush out of spawn points thinly disguised as buildings – they're literally coming out of the walls – and that kind of dehumanisation is troubling. Zombies get around it. Their implied and understood behaviour is in line with the tropes of video game structuring.
The zombie also presents a twofold challenge. You can kill one but since they don't have a gun – since they are slow and ungainly – you're tempted also, to dodge them and avoid the encounter. So as much as they are cannon fodder, zombies are also platforming puzzles. They're like Goombas in Mario, which you can either bounce on top of and kill or try to jump over. In most violent games, the player can only progress once they've shot everyone.
With zombies, there is a choice, especially in Dead Rising, Zombi and Resident Evil there is a small, added nuance to each encounter. It's only in an instant, but letting the player decide between fight or flight when faced with the undead is a microchosmic example of choice. Something with which both games and game designers have become infatuated.
So zombies let games dodge moral questioning, straightforwardly implement constant action and killing, and offer choices to the player. By that measure, they represent some of the medium's worst excesses: refusal to engage in any kind of introspection, conflation of engagement and interaction with violence and spectacle, and giving precedent to the audience rather than the creators. It's plain to see why zombies and zombie games continue to exist, why they characteristically refuse to die, but it'd be better if someone shot them in the head. Otherwise, we'll all end up just shuffling aimlessly around.