In the movie I Am Legend, Will Smith plays a man who lives in fear and solitude for about three years. He is driven by his desire to fix the biological disaster that seems to have wiped out all of humanity except himself. He borrows his life-motto from Bob Marley’s mission in life: to “Light up the Darkness.” He wants to be as dedicated as Marley in bringing good, or light, into the world.
One obvious spiritual theme in I Am Legend is Light vs. Darkness. God designed us to depend on light. Light implies reconciliation, goodness and purity. We dwell in the light if we have nothing to hide. Jesus even calls Himself the Light of the world. When we dwell in the Light, we can see and accomplish a lot more than when we stumble around in the darkness. When we dwell in the Light we live unhindered by all the blindness and fear that comes from staying in the dark.
In I Am Legend there are two kinds of people left in the world: Light-dwellers and dark-dwellers. Will Smith is one of the few light-dwellers left in the world (for a while he thinks he's the only uninfected person in the world). The vast majority are crazed zombies who can only come out at night because sunlight will burn them alive.
We would like to hide in darkness to cover up our shameful lives. But Jesus shines light on our sin, exposing it... it's no wonder that we naturally hate Him. We hate the Light because He exposes our evil deeds for what they are.
Sin is similar to the infection depicted in I Am Legend. Sin has devastated humanity. Sin reduces people to animalistic impulses. Sin inspires people to hide in shame. Apart from God, we are enslaved by sin to the point that we can’t help destroying ourselves as well as the world around us.
In I Am Legend, the majority of people left on earth have the capacity to be productive but they are compelled by their infection to use their physical and mental facilities for killing, destruction and infecting others. The minority, the light-dwellers, risk their lives in their attempts to save the dark-dwellers.
Toward the end of the movie Smith emerges as a Christ/savior-figure to the raging zombies, holding a vial of his own blood, which would cure them if they would only accept it. Smith culminates the Christ-parallel by giving the blood away and then sacrificing himself in order to make the blood available to other people. Smith also resembles Christ in that he was the only person in the whole city who was immune to this horrible infection.