At least, the original ending of I am legend, viewing this ending and reminding the somewhat awkward cinema release explains a lot more about the relation between the "zombies" and Smith's character. There is an interesting conversation at Firstshowing with lots of comments (as of now hundred) and more info about the controversial ending.
Video: I am Legend Original
This end states clearly that Neville is the monster, hence the 'I am legend' title. Without this ending the title and film makes no sense at all.