Warning: Spoilers the only part I remember is the end

I can’t tell you exactly when I seen this movie. All I do remember is a group of people are in a building for protection aganist a mist/fog like substance. At the end a man is his car I think he tried outrunning it but failed. He doesn’t want his son to die a brutal death so he shoots as he sleeps. As he is about to and his life, the mist-foglike disappears. Now he is devastated because he killed his son and if he waited afew more mere seconds they both would have survived.