Will Smith's return for I Am Legend 2 only reinforces the original movie's biggest error, years after its release. I Am Legend was the third adaptation of Richard Matheson's totemic horror novel, which inspired everyone from George A. Romero to Stephen King. The novel's simple but ingenious premise pits the last man alive against a world full of vampires, but while the Will Smith adaptation used the book's concept, it changed a lot, including dropping the grim ending. In the book, protagonist Neville realizes the vampires see him as a monster, as he's been staking them in their sleep, and that they are the next step in evolution.
https://screenrant.com/i-am-legend-2-movie-original-ending-biggest-mistake/