The more I watch this movie, the more convinced I become that this movie is less about the mist as a horrific result of military experiments gone awry, and more a commentary on the fear of death and the unknown beyond.
The mist and its Lovecraftianesque creatures are symbolic of humanity's existential dread and the abject fear that there is quite possibly nothing else but this, and the end here is the end forever; or even worse, that what lies beyond is not a benevolent Creator, but rather an entity that either is indifferent to our existence and our plights, or is actively malevolent against us. Jean-Paul Sartre described this fear as "existential nausea."
Mrs. Carmody's ever growing religious cult was the very opposite extreme. She proclaimed to be a prophet of the one true God, yet was hypocritical, judgmental and quick to cast stones even in light of her own sins. Her fundamentalist extremism insured they never escaped their box. But what seemed the logical end result of her many violent transgressions only served to martyr her in the eyes of other extremists -- as it so often does -- and serves, in their twisted minds, as vindication of what they've been preaching all along.
And there are those who simply did the best they could with the circumstances they were given, but in the process, did not sacrifice the wills of others for their own gain while simultaneously giving of themselves unto the service of others. No one can say with absolute certainty what lies in wait beyond this life, but we shouldn't squander the moments we do have on this Earth entombed in fear and letting that fear manipulate us into becoming the monsters we sometimes create ourselves.
The end of this movie has always been, in my opinion, the most tragic. The ending was almost a happy one, if only they'd held out for a few moments longer. But in relation to the underlying theme laid out above, this group of people's break from the store to the car to the literal end of the road was metaphorical of venturing out into the unknown, even if it means leaving the comfort yet unstable illusion of safety and well-being, to take our chances navigating life the best we know how. We may feel as if ultimately the only thing we have any control over is when and how we die, but even this is just another illusion.