Not sure why we assume they are "creatures"....SPOILERS
It seems much more like an affliction or phenomenon than a bunch of physical entities doing this. I'd be more inclined to assume people are having some kind of reaction to a virus....or something in the mist...or even a magnetic field or the radiation/ultraviolet light of an unidentified nature, or poisonous gas.....than assume "creatures" are driving people mad. For example, if a bunch of creatures were all around, wouldn't Malorie and the kids have been much more..."vulnerable" to a physical attack while drifting down a river, blindfolded. Or...while walking through a forest, blindfolded. I didn't read the book, so maybe there's more of an implication that they are indeed physical beings creating this mayhem. But it sure is not made clear in the film.
Also...there's no rhyme or reason to why SOME people are impervious to the self-destruction effect, and instead just go around like psychotic "recruiters", trying to get people to look into the sky.
Also, as another poster pointed out....the older guy in the suit that they let in....was apparently one of those crazy "recruiter" people. However, he was able to act normal around the group and conceal that he had been affected....until such time as he was ready to make his moves. That's yet another inconsistent (or least unexplained) wrinkle.
Also, early on....it's KIND OF implied that whatever it is people look at....is personal to each individual. For example, the lady who was John Malkovich's wife....spoke to her mother when looking at the vision in the sky. However, the writers didn't do much more with that particular premise, that I can recall. Except I do seem to remember "voices" beckoning Malorie while she was looking for the 2 kids in the forest--trying to trick her?
Having said all this, I did enjoy the film. It was good and spooky (and TENSE). And Sandra B. really committed to the role. It was good. Could have been GREAT.