The original story was left unresolved, as a lot of King's short stories are. Many of them are just ideas that pop into his head which he then throws at the wall, and sometimes they only stick for a few minutes - not long enough to warrant its own book, but intriguing enough to put onto paper. This is one of those stories. So if it was to be made into a movie, it was going to need an ending.
Despite having countless bestsellers to his credit, relatively few of King's stories or books come close to matching that success when made into movies. The person who developed this screenplay basically took the original story and gave it a twist from his own dark mind, and the result impressed even King. In the end it reminds of the Twilight Zone episode where the residents of a small town neighborhood experience a power outage, and they resort to panic and finger-pointing when faced with the unknown. Add a major religious edge, and you've got the makings of a tense but still believable (or at least imaginable) path towards a complete breakdown.
Many times after seeing a movie, you get in the car and start talking about what's for dinner, where you're going next, work tomorrow, etc etc. After seeing The Mist, when you get in the car you'll be talking about The Mist.
See this movie.