For anyone paying attention, The Thing is teeming with hidden clues and subtext; for example, did you ever notice that Windows drops the keys during the scene where Bennings is infected, how the intruder is later able to destroy the blood supply?
John Carpenter feels the movie worked so well because he had a long pre-production period on it, allowing him to really think about the script. His biggest addition was turning the movie into one long, very destructive game of chess between MacReady and the alien. This is established in MacReady’s introduction, where he plays computer chess and feels certain of victory, only to get beaten at the last moment. And what does he do then? He destroys the computer rather than lose.
The movie then plays out like a chess game, where teammates – or pieces - get taken off the board and MacReady is constantly losing the upper hand. When he realises the alien has essentially won he destroys the camp to rob it of victory. And then, in the last scene, he doesn’t know if Childs is infected or not, meaning the game ends in stalemate; no wonder he looks so exhausted.