TL;DR version:
The movie is the latest Hollywood adaptation of a novel about a lone human survivor of a global pandemic that killed most people and turned the rest into vampire-like creatures. In this story, the protagonist eventually learns that the vampires he hunted by day weren’t all the mindless monsters he assumed them to be; many are fully sapient members of an emerging new society. The title comes from the protagonist’s bemused dying realization that this new society will likely share legends of the monstrous day-walker just as our society shared legends of monstrous vampires.
The original version of the Will Smith movie makes a nod to this source material when his character realizes the infected people attacking his home are just trying to rescue the young woman he abducted for medical experimentation (especially since his experiments have already killed a number of others). Understanding this, he lets her go and the infected leave.
This ending was deemed unacceptable to the studio because peaceful coexistence with one’s enemies is less cinematic than blowing them the fuck up.
Hollywood has taken a few stabs at this story already, but the 1964 Vincent Price movie The Last Man on Earth was probably the adaptation that stuck closest to the source material.
