So by this it can be assumed that it could be possible that I Am Legend could be taking place in an alternate New York in the universe of the vampire film Daybreakers. Basically it goes like this, events that made Vampire!Humans in Daybreakers and Legend are fairly similar how ever the storys view splits after this. In one Vampires become like a overwhelming Zombie Apocalypse with only Neville able to save us all while the other is where while humanity becomes Vampires society has changed very little. People still go to work, raise families, drive cars, and make and watch Fox News. Basically the world and civilization in fact hasn't collapsed only "changed".
However Neville (due to his mental snap(s)) doesn't see this and is now only a raving lunatic driven by his own hatred and fear of the new Human!Vampire society. Essentially Neville is a Bigot in the most fantastical sense of the word. He most likely killed his own family out of fear they would become "freaks" like the rest of society. Afterwards Neville began to suffer from severe survivors guilt and mental exhaustion in where he began to break or disassociate himself from reality. After this Neville built a delusional fantasy world in were Neville is a savior of sorts in where Vampires are zombie like monsters and he is the (seemingly) last sane and non mutated man left on a ruined Earth.
However in actuality Neville is nothing more now then a serial killer and madman who has probably killed more people then anyone else in New York's history thus making him a Legend.
Although this does leave some questions that the novel would leave somewhat unanswered but can be explained such as...
- The reason Neville appears completely alone most of the time is well he is outside Only during the day so most of NYC's populace (being vampires now) would be asleep in their homes. This would be no different then a actual intruder breaking into someones house and stabbing them while they slept.
- Neville has probably killed more of his fair share of Subsiders (starving Vampires turned into Bat-like hybrids) so this would further confirm Neville delusion that all vampires Are bloodthirsty (no pun intended) inhuman monsters.
- The novel takes place in Los Angeles, not New York. New York was used solely in the third adaptation of the novel in which Will Smith stars. In The Omega Man, it was set in Los Angels. In The Last Man on Earth, the city was never named nor shows noticeable landmarks to identify where it was (primarily because that film was shot in Rome, Italy).
The book speculated that Neville was bitten before the story began, by a bat that carried a weakened form of the bacteria, developing his immunity.
- That was in the Price version too.