Turns out there is shit in the fog and it's insect like things. The people (including the protagonist and his child) who are stuffed into a grocery store make a run for it and pretty much escape the immediate area... only to run out of gas. In the middle of nowhere surrounded by ridiculously thick fog and insects they figure their death wasn't too far away and decide to take their lives. Inspecting their revolver they find one problem: there's four bullets and five bodies. The protagonist of the movie decides that he'll be the one to be eaten by the creatures and to have mercy on everyone else. He shoots everyone in the car (including his child and yes, it was consensual, at least amongst the adults). Obviously distraught over just shooting his fucking son,he gets out of the car to embrace his fate and... well, the military shows up annihilating the monsters and escorting rescued civilians to safety. The end.

The Novella ends with the hero and his son and driving away through the mist toward Hartford. He thinks he hears one word as he checks for stations on the radio. That word is "Hope." Obviously, the film version needs a bit more resolution. I'm fine with the five people in the car. It should have happened this way: they hear a monster approaching. They discover they have only 4 bullets. They resign to their fate and he cocks the gun and is about to fire. Suddenly, the "monster" reveals itself to be an Abrams tank and they are saved. The entire movie, he is willing to fight, fight, fight to stay alive. Anything to save his son, then at the end, he shoots his son with no monster in sight? They had the gun while in the grocery store. He could have ended it then. Why didn't he? Because he'd do anything to stay alive and to protect his son. He would have waited until the monsters were slamming into the car--he would not shoot his son just because they ran out of gas. Also, his son woke up and looked at him as he aimed the gun. Then he shoots. THIS CROSSED A LINE. I've never seen a man kill his child onscreen in this way before. It was inhumane and disgusting and despicable. Unfortunately, we live in a time where people are desensitized to violence and don't recognize something as outrageous as this for what it is. These are the people who call others who still have some humanity a "pussy" for daring to have a conscience.

Alright. I can understand your explanation as to why the ending might not have been as good as the novella's. I actually made the same comment about him being a little too quick to the trigger especially since they were willing to fight through so much just moments before. That argument I'll take, but I still wouldn't have preferred for the movie to have ended any different.