It seems like one of those movies that started off as having an awesome script. Then, to sell it to the big studios, the penny-pushers requested some changes to include "what's popular now" and to suite a "broader demographic", and that's what added weird shit to the original script. Thus you get the feeling of watching two or more stories meshed into one - which is indeed what's happening.

I got the same feeling watching Hancock and I am Legend (by chance both Will Smith movies) and I know for fact that I am Legend got meshed with the Richard Matheson title just to up it's popularity.