First of all, they did not kill themselves at the end. The father killed all of them and was going to kill himself but ran out of bullets. Second of all the military did not save anything, as crazy as it may sound, the bible lady was right in saying that the son had to be sacrificed to end it all. Third of all it clearly states the reasoning behind the mist. Fourth of all to create atmosphere and wonder, the monsters are not fully seen intentionally. Lastly, the ending in the movie was not in Stephen King's book, however King congratulated the director on the ending saying that if he had thought about that he would have put it in the book.