Prepare for a potentially long winded post.
I'm a very big fan of the original story and have read it many times (it's pretty short). Early on with the production of this film I had really high hopes that it would stay true to the source material, though as time went on I felt that less likely. When I saw the film I was pleasantly surprised by the first 2/3rds of the movie (as it seems most people are). I, like many others, was disappointed by the original ending in IAL. The only aspect of it that was true to the story was the fact that Neville dies. And frankly, the fact they attached the hopeful ending with the woman and child finding refuge and referring to Neville as a Legend because he 'saved' mankind with his cure was a slap in the face of the title's original darkly ironic ending.
(Brief story synopsis) In the story, Neville does come in contact with a woman who appears to be human, but is actually an evolved "vampire." Neville is soon captured and tried by the evolved "vampires" and killed for his crimes (going out in the day and murdering helpless "people.") He realizes as he looks at the faces in the crowd of his execution that the people can barely look him in the eyes because they are terrified of him. He has become the boogeyman, he is what goes bump in the night (or day
) and kills them while they sleep. It is in that regard that he refers to himself as Legend, he is the last human, soon to be killed, and will only be remembered as this Legendary monster.
Now, that said. I did like the alternate ending more than the original. The main reason being that as he realizes the desire of the mutant (wanting the woman) he takes her out to him. As he looks at the wall of all the Polaroids of his "victims" he realizes what he's done. These mutants are in fact sentient and he has been experimenting and killing them. It's almost like the wall of a serial killer or some such. Closer to the book in this regard IMO. However, he lives in this ending along with the woman and child, there is no real message of hope however, just an uncertain future they are moving forward into. I was more pleased with this and the other ending.
Now...if I had my druthers and was able to make my own ending it would have been thus...since the movie introduces the woman and child, might as well keep them in it after the death of Sam, however, after him becoming more accustomed to having people back in his life, they should have been killed by the mutants, making him feel even that much more vulnerable and alone. They could keep the alternate ending, having him walk out with the woman and giving her to the mutant...at this point, after having viewed the pictures, realizing what he's done, and having lost the will to live, the mutant gives him some indication that he must go with them, not forcing him per se, but he realizes he has to pay for what he's done, have Neville go with them and have them "execute" him in some fashion. This ending, I think, would remain true to the spirit of the ending in the book. As far as being a really bleak movie / ending, yeah, it would be...but it was a pretty bleak story anyway.
As far as the scene with the trap that Neville falls into, yes, I'm pretty sure the mutants set it for him as well, displaying that they're actually sentient creatures.
Phew, I think that's the most I've written in my entire time on these boards, lol. I'll edit it later.