The whole problem with I Am Legend (and yes, there are spoilers here) is the third act. It actually kinda went to hell.
I have a fix, in case they want to re - shoot it to make it better. Interestingly, this fix may have already happened in the development process because hints of it are in the movie.
OK, so... we have to get Neville's "sacrifice" to make sense.
We don't really get a sense of how obsessive he is about saving the city in the first act. I know we have him talking about that in flashback, but we don't get the sense that's what's driving him today while he's out hunting deer. Actually, that solve is difficult. I'm going to ignore it for now.
Here's a thing. Did you know that the night-seeker on the table had a character name? "Alpha Female". And do you know what the name of the main night-seeker dude who is Neville's nemesis is? "Alpha Male". I think that at some point it really had to be more clear that the reason Alpha Male was going after Neville was because Neville has Alpha Female hostage (even though Neville, of course, doesn't see it that way.)
Also, very briefly, you see just how many people Neville has killed in his quest for the cure. There are pictures of them on his wall. This is a big Moral indicator (and indicator that he must eventually die in order to pay for his sin.)
It would be nice if he learned Alpha Male could actually talk. That's always a cool and super-creepy moment in any film. Anyway:
So at the end -- what if when the three of them made their way down to the lab they found Alpha Female was really cured? As in, she's just like them now? And scared? Well, now you don't have the problem of "Why doesn't Neville just go into the chimney with them, tossing the grenade just before slamming it shut?" problem. And what if it was his blood that cured the night-seekers? And that by letting them attack him they would get the cure which would virally spread through the hive?
I know, it would require a few more lines of dialog and an extra scene or two to re-jigger the plot to go that way, and I certainly don't think that's the only solve. It's just a solve. Neville allows the night-seekers to kill him and in doing so cures them and saves the world. He would then be Legend indeed.
Sometimes I'm stunned by my complete ignorance of things so basic to Western culture as to be part of our visual canon. Lotte Reiniger's Prince Achmed is achingly beautiful. It's a shadow-puppet feature from 1926.
Arguably the first feature - length animated picture, with a distinct art deco style, it is beautiful, erotic, exciting, wonderful -- everything you could ever want in a movie. The figures have expressions... I'm not even too sure how they do it. Your mind. Will be blown. And hey, kids! It's cheaper and safer than taking drugs!
My sister is going to love the creatures in this movie.
Also: I just got the Ultimate Super - Duper Edition* of Blade Runner. I'm going to watch all five versions! Good thing I'm rendering mostly!
*not the official title