Any zombie movie has the potential to illustrate the ugliness of sin. Zombies are formerly pleasant, hard-working, mostly honest people who turn into animalistic, blood-thirsty goons. Apparently, anyone has the potential to be a zombie. Resident Evil, for example, explains zombification through latent brain waves that give the body enough energy to keep walking around, except only as senseless beasts. According to Resident Evil, people without restraints become monsters.
Zombie movies are ideal for scaring people because they portray an everyday world gone wrong. The people you thought you could trust are now clambering to tear out your larynx with their teeth.
I Am Legend does a particularly good job of demonstrating the spiritual implications of such a biological disaster. There are hints throughout the movie that keep God in forefront of the plot.
(Warning: The following paragraphs might spoil the film's plot if you've never seen it!)
In the beginning of the movie, Will Smith defends God's reputation, saying that people imposed this disaster on themselves. Later on, after he endures a few years of loneliness and fear in a civilization devoid of civilized people, he struggles with this conviction.
Toward the end of the movie a young Cuban woman saves his life and claims that God guided her to find him just in time. She goes on about how, if we only listen, we will hear God's voice. Out of pent up frustration from years of fruitless work, Smith shouts back to the woman that God doesn't exist. Before the end of the film, however, Smith thinks over the young woman's words and comes around again. In fact, his faith in God's divine plans is so dramatically restored that he sacrifices his life so the woman can escape with her son and the cure to the disease.
It is realistic that a man in Smith's situation would struggle with his faith. The fact that his remarkable experiences test and eventually solidify his faith makes the whole ordeal worthwhile.
A recurring theme is, "Where is God when bad things happen?" The answer from I Am Legend is, "We should not blame God for our mistakes." There are posters in deserted city scenes that say "God still loves us, do we love Him?" Another poster portrays of Michelangelo's Sistine Chapel, except where the man's hand used to be, reaching back out to God, there is a gun pointing back at God.