The traditional zombiefilm is about harsh, immutable reality crashing full-force into the facade of human emotionalism, misconceptions, and escapism, and necessarily obliterating them. The humans band together in sleepless impotent flight as the unexplainable monsters surround them, wailing at the sight of the unfathomable horrors--as if such pitiful pleas would stir the zombies' dead hearts. All the loving they can muster to shield each other creates a smokescreen that blinds them to real dangers, and the zombies stroll through effortlessly. Don't kill him, he's a human, you must love him... even if he's been bitten and will soon turn. The only way to prolong life--temporarily--for self and loved ones is to plan ahead, not confuse emotions for thought, and always keep fighting. Fools die quickly, but ultimately, all are mortal.
Zombieland inverts this. In Zombieland, the protagonists roam aimlessly, chasing simple pleasures as they stumble upon them. Zombies are the background, the focus is the humans and how their weaknesses and failings are ok because they're nice people who love each other. Murderous intent is easily forgiven, because to do otherwise would be mean. And humans aren't mean: zombies are! Zombies are the jerks that cause all problems, we're innocent. It wouldn't be fair if they attacked and killed us, so that probably won't happen. Plus, we love each other really hard, and that helps. Life's a game, it's ok, relax. Here, smash this stuff to make you feel better. Don't be so uptight, buddy! The people we love will never die.