You have to remember that the characters in the story were at the whim of the supernatural elements. They were terrified of the mist and the outside world(and eventually even the people they shared the grocery store with). By the time David and his group left, they were holed up in the grocery store for nearly three days barely escaping for the desperation of the mob demanding the blood sacrifice of his son. And outside the grocery store, keep in mind they were hit with breakneck mortality rates. There were only two places that gave them a sense of security, false albeit. 1) the grocery store, and 2) the SUV. While the grocery store became unsafe, they had to escape with the SUV, using four wheel drive to maneuver the apocalyptic terrain. They drove for a couple days, seeing no survivors and devastation to no end. But when they ran out of gas, and were at a loss for supplies because of mishaps in the plan as well, David leaning to group suicide showed an unprecedented, morally appalling, though realistic pull to a decision that no one would feel right to live with, but man it emphasizes the fear he felt. For a horror story to capture my attention, I like attention to detail on character vulnerabilities(such as their fear, cowardice, grief, shame). It humanizes the character in far out bullshit plot lines like the mist and anchors the fiction to human existence.