The goal of a horror film is to strike fear in the heart of the watcher. Audiences react in horror to a grotesque scene or a surprise jump in a dark hallway. These fears only last as long as it takes for you to get your scream out. (or jump or whatever you do) However, there is something about zombie movies that comment on the human existence. The zombie is man stripped of whatever makes him human. All that was of value is lost and only the drive to eat remains. While this is make believe, in real life people are afraid of losing themselves. Certain proclivities that a man allows have the potential to overtake him. Man becomes a slave to addiction. (drugs, alcohol, porn, etc.) In pursuit of that one thing families are destroyed, futures are destroyed, children are hurt, and jobs are lost. Everything that was once loved and cared for is gone. The man himself loses who he is. After a while he might come to his senses, but the pieces of his life that formed a coherent whole are lost. He must now pick up what is left and form a new whole.
A good example of this is Jack Torrance in the movie The Shining. His addiction was alcohol. When he drank he became a different person and his anger poured out of him destroying everything around him. He tried as hard as he could to control himself. He stopped drinking and got help. But after taking a job as a caretaker for a huge hotel in the off season, Jack withdraws into his own world in which his family turns against him. In reality they love him, but he sees them as if looking through a tainted window full of betrayal and deceit. Soon he turns against them. Somehow he finds alcohol in the abandoned hotel and acts on every fear within him. His wife and child barely escape with their lives. As they leave, he comes to himself just for a moment, enough to see that any vestige of life that might have remained within him is hopelessly lost, and in the culmination of his addiction he dies.
The potential of this degeneration lives inside of every man. Like fighting an enemy, we try to keep the evil at bay. Time goes by and weary we give up the fight. Evil then takes what we care for the most and dashes it upon the jagged edges of addictive behavior. Broken, we look at all we lost wishing to God we had kept up the fight when we had the chance.