Why does "I Am Legend" have two different endings?

Because the movie is divided into two versions: the regular version and the extended version. The ordinary one is that the protagonist dies and the two survivors go to a base that is not infected. The longer part is that the protagonist escapes from death and continues the journey to find the survivors with the two survivors he rescued.

Ending 1: Robert, played by Will Smith, extracted the serum from the recovered "Night Stalker" and gave it to Anna and the little boy, allowing them to escape from the tunnel. And he detonated the grenade and died together with the Night Demon. Anna took the little boy to the base where humans gathered and kept the serum.

Ending 2: Robert pushed out the night demon that had gradually turned into a human form, injected it to restore it to the night demon, and returned it to their boss. Instead of killing him, the Night Stalkers left. The next day, Robert and Anna left their residence to search for other humans. Extended information

Overall plot:

Ignorant human medicine invented a new virus that can cure cancer. Unexpectedly, the virus mutated and turned infected humans into living zombies. A sudden outbreak of a large-scale virus spreads so fast that almost no one can stop it. No one knows where the source of the terrible virus started. They only know that it cannot be stopped, irreversible, and cannot be cured. In the end, The important thing is that it was created by man.

Colonel Robert Neville is a talented pathogen scholar in New York City, serving the U.S. military. As the vanguard of the government's investment in researching vaccines against epidemic diseases, he was helpless as he watched the virus raging on the streets.

This airborne virus eventually enveloped the entire city at an irresistible speed, forcing the president to order a blockade of Long Island so that uninfected people could leave the island. It is conceivable that this approach caused great panic among the citizens, and New York City suddenly became chaotic.

Robert anxiously sent his wife Zoe and 7-year-old daughter Mary onto a helicopter out of the city, only to witness the tragic death of them in front of him. At this time, death may be the best choice, because those who were infected but did not die had their bodies mutated and became terrifying creatures, living in the darkness and shadows underground in the city, avoiding the sun-- They have the most primitive desire for fresh blood and flesh.