Question about ending

Rating: 5/10 (17 December 2007)

Let me first say I was disappointed in the movie, I guess b/c of all the hype that surrounded it. I saw it in IMAX and actually thought the new BATMAN trailer at the beginning was better than Legend.
The question I have concerns the ending of the movie, you see the woman and kid drive up to this small town surrounded by15 foot concrete walls and inside there is a community of people who have survived. What I don't get is during the movie you witness these mutants climb walls and hop up light poles and pull them down, why wouldn't they just climb this wall? And it's not like they are isolated, you can clearly see that the community is just outside an existing community, if those people died and turned into zombies or whatever wouldn't these people be easy pickings? I also don't understand why Smith had to be heroic and blow himself up instead of just climbing in the safe/chimney with the other two and just throwing the grenade? And why would that one grenade blow up all the zombies when the other HUGE explosions couldn't? To many questions left unanswered. Will Smith does a terrific job of acting but the story left much to be desired.