World War Z showed us that Brad Pitt can run like Tom Cruise.
The lesson is always the same: When society breaks down, the remaining humans treat the world as a nonstop cannibalistic rape festival.
But Actually ...
Ask yourself this: If humans naturally become murderous sociopaths in the absence of modern society, then how did modern society ever form in the first place?
"Man, I am so bored, there's nothing to do ... oh, wait, I got it: civilization."
The whole freaking reason we have a civilization for the zombies to destroy is because humans are naturally cooperative. You don't need to take our word for it -- studies performed by researchers at Harvard and Yale found that our basic human nature dictates an overwhelming need to cooperate with other humans, even if said cooperation results in some measure of harm to ourselves. Biologists say the same thing. Despite what the self-checkout line at Harris Teeter may have led you to believe, humans actually give a decent-sized shit about one another.
In recent regional crises like Atlanta's blizzard-induced traffic gridlock and Hurricane Sandy, examples of basic human kindness weren't difficult to find. When serious tragedy hits a community, most people's first impulse is to see what they can to do help their neighbors, rather than to carry all of their canned food into the basement and start loading their shotguns. It's not even because we're nice guys -- it's because instinctively we know that we might need that person down the road. So even if some worldwide crisis were to transform us all into selfish mutants, the theory of reciprocal altruism suggests that we'd probably still be willing to share that can of beanie weenies if it meant we could get something in return later, even if that something is just "an extra person I can feed to the zombies to make my getaway."