But the hero dying at the end, and so passing into Legend, was the point of Matheson’s story. I think the earlier version, “The Last Man On Earth,” starring Vincent Price, made that point more clearly than “Omega Man”.
Vietvet
In the book, the "altered" humans are more readily identified as vampires... unlike the "mutant" angle used in The Omega Man. The "Legend" part comes from the protagonist realizing at the end that he had become the new legendary monster (viewed as such by the "rest of the world").
It's been years since I read the book, but I think that's the point where it ends. In The Omega Man, the death scene is added to give Neville a Christ-like dimension (he dies protecting others, ensuring that he provides the means (serum taken from his own blood) to protect them from the plague.
I can see how the latter ending would be more appealing to writers and directors; the original ending is a bit hard to get across on film. It'll be interesting to see which way this new film ends.