Zombies fascinate me. They confuse me. What drives them? Rage, hunger, these are the frequent explanations. But they are insufficient. They do not answer an essential question: Why do zombies not fight each other? There are not only motivations for their violence, but also something that motivates them to not be violent.
The true definition of a zombie is not a risen corpse or a person with a virus that makes them insane. Instead a zombie is most simply described as a human which no longer defines itself as part of humanity.
These could be traditional mindless hordes wandering slowly with their hands out. Or they could be nearly-human but light-sensitive as in I Am Legend. Recent years have brought about the fast zombie, along with widely varying levels of intelligence. But they all share two traits: they are our enemies and they do not attack each other. There are a few rare exceptions, but as a general force, they are non-hostile to each other.
We could apply this to real life, identifying those groups of humans who consider themselves to in some way be superior to the point of separation. These could be the Nazis who thought they were descended from gods, in opposition to the in some way inferior Jews and Slavs. These could be Islamic terrorists who feel that our refusal to join them indicates that we are fundamentally flawed: they see us as the zombies.
I think it goes the other way, actually. Zombies are like so many supernatural phenomenon: cultural representations of fears. In this case they represent the fear of being corrupted, of betrayal, of an existential threat to humanity. We're a frightened species, thanks to our imaginations. Zombies have changed over time, once being a form of slavery, of a person of supernatural powers turning us, making us do their work. They were not something we'd expect to see forming rampaging hordes, though necromancy was around as a fear. Over time they've become less personal, transitioning from something that happens to us to something that happens to people, to the point that they are viewed less by their corruption from humanity as their threat to humanity. Maybe this has some meaning, a way to measure changes in social attitudes and fears.
I really should check my town's ordinances concerning shotguns.