Where do the monsters come from?
Not that it would affect the movie one way or the other, but I found myself wondering just what were those creatures and where they could have come from. The most obvious answer is that they were aliens, but it didn't look at all like they were the kind of being that would be capable of building interstellar ships (or building anything, for that matter). How can a blind creature develop any technology? They seemed to be just brainless killing machines.
My theory is that an invading intelligent alien species brought those beings to earth in order to exterminate the human population without having to fight a war themselves. Let a number of those things loose on the planet, and they will do all the killing for you.