One of the biggest questions of the film. We will first break down the sequence of events and then explain the motives behind each from a human and Thing perspective.
1. Blair runs the computer program and discovers the possible threat the Thing poses.
2. Although we don't see it, Blair kills the dog and destroys the tractor.
3. We see Blair destroy the helicopter.
4. Blair injures Windows and destroys the radio.
5. Blair is locked up outside in the shed.
6. The next time we see Blair, Mac is questioning him as to the whereabouts of Fuchs.
7. The next time we see Blair, he has clearly been assimilated: he is building a ship, attacks Garry, etc.
Now, the question is asked, why would a human Blair kill the dogs and destroy everything? At this point, it is very clear that Blair understands the situation and feels that his comrades do not. He is afraid that the Thing will escape and infect the entire planet. He kills the dogs, destroys the helicopter, and destroys the tractor to isolate the men. Although Blair says, "no dog can make it a thousand miles to the coast," he was more or less referring to a dog on its own, and not a team with a master. He may have also killed the dogs to prevent infection, though we know that an axe would not be sufficient enough to prevent that - keep in mind that Blair has lost it at this point. Blair destroys the radio so that no outside help can be reached; it could be another way out. If this was the case (that Blair was assimilated after his lock-up), it was either Palmer or Norris that got to him. Somewhere during the 48-hour period that Mac refers to on his tape recording would seem logical.
The Thing perspective: The Blair-thing would have destroyed the tractor, radio, and helicopter to prevent the men from escaping and to keep them from reaching outside help. The dogs were killed to make his motives seem more human. The Blair-thing's plan worked; he was seeking isolation so that he could build a craft to escape. This makes it look as though the Thing's intentions are not to assimilate the entire earth, but to simply get off the planet; had the Thing wanted to assimilate the entire planet, it could have just as easily stolen the helicopter as Palmer and fly to the nearest base, country, whatever.
Something that comes into play that suggests Blair was assimilated before the lock-up was the tampering with the blood. See below under "Who got to the blood?"
Added 09/14/07: John ("The Ultimate Thing Movie Lover") I've seen this movie a thousand and one times and know it line for line. I don't know if anyone still reads this post, but I'll tell you what I know about this issue:
Blair was infected during the autopsy. It was a cellular infection which took some time to gain control of his body and "conscious" mind. The Thing gained more control as the cellular infection increased over days. We know this is a very strong possibility by the computer simulation that Blair ran which demonstrated the Thing cell taking over dog cells. As the infected individual's body is slowly taken over, they remain conscious during this state but unaware that they are infected, but their actions are subtly influenced by the Thing's ever-increasing hold over them. Blair destroyed the dogs, tractor and chopper. The dogs being killed was a completely human response backed by his scientific knowledge that any one of the dogs could have been inadvertently infected during their attack in the kennel cage by hostile or cellular infection. In his destruction of the tractor and chopper, the Thing influenced his decisions, and I believe it was subconscious.
*Think about it...The Thing was a masterful strategist. It had been alive for eons and assimilated hundreds, maybe even thousands of beings and intellects. Many of its actions had a two-fold and even three-fold win-win situation that still gave it an edge in the final game.
Yes, he destroyed the tractor and chopper to prevent the infection from spreading to populated areas (human response), but he also destroyed them to hide the fact that he had stolen parts from both.
The crew was less likely to investigate a smashed, ripped-apart tractor or chopper to see that all of the parts were there. Blair was well aware of military protocol and knew that his mental breakdown would call for them to isolate him, giving him time to assemble the parts and build a craft to get him to the nearest populated area. The craft was never meant for long distance travel.
A third goal in destroying the tractor, chopper and communications was to prevent anyone from getting out until they were ALL infected. Military protocol would have been to capture and quarantine all parties involved until it was determined who was human and who was not. The Thing could not take the chance of someone contacting the rescue team and revealing the alien infection. However, Blair was not the only one who may have been infected by cellular means during the autopsy... Fuchs may have also been infected!
We see Blair performing the autopsy and Fuchs "sniffing" and going through the clothes and other items there. Those garments and items are soiled with possible dried blood and other body fluids. Fuchs is seen standing there listening to Blair while going through the items without gloves or other personal protective equipment.
Since the scene ends this way we can assume that Fuchs assisted Blair in other ways during this autopsy even if it was just "Hold this" or "Move that." Cellular infection is HIGHLY probable at this point.
This theory would also explain Fuchs' disappearance. By the time he talked to Mac, he knew that cellular infection was a strong possibility. Blair at this point was isolated and locked up (or so we thought), but Blair was secretly moving about the camp collecting and assembling the last of the parts and tools he needed to finish his craft. It was Blair who killed the power and walked past Fuchs' door in the dark. By the time Fuchs got outside to chase this suspicious person, he found Mac's undergarments.
Now remember, if cellular infection was possible, then by this time Fuchs' actions would be influenced subconsciously as well, since he was infected during the same time period as Blair.
Thus, after finding Mac's undergarments, Fuchs heads to Mac's shack, possibly planning to snoop around for more proof that Mac was infected before accusing him publicly (clearly a human response). While there, he stuffed Mac's shorts in the oil furnace, this being a subconscious influence by the thing to keep an ally hidden.
In all likelihood, Fuchs began to question his own actions on the walk back to the main compound and came to the realization that he "must" be infected to have just done such a strange thing rather than exposing Mac.
*Now remember... the Thing only attacks when discovered or "attacked." Why wouldn't a self-realization spark an attack as well?
At this point during Fuchs self-realization, the thing begins to reveal itself, going for a more quickened "hostile takeover" of Fuchs rather than the cellular option. As Fuchs begins to change and lose the last of his humanity, he does the only thing he can, setting himself on fire in the snow...
For questions in regards to this response, mail Journeygod@yahoo.com
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Another hint that Blair has been infected before he destroys the radio and communications equipment is that his intentions, as 'the thing', are to destroy any means of communication (radios) that will allow the remaining humans of the research outpost to contact the outside world and warn them of 'the thing' that they unleashed from the ice.
Blair, as 'the thing', could prevent the humans from warning the rest of the world to either proceed with caution when approaching the arctic facility, or to isolate or even destroy the facility from afar. Once Blair, as 'the thing' was completely sure that the existence of the creature was safe and only known to the remaining humans in the facility, 'the thing' could take as much time as it needed to slowly and surely assimilate the rest of the crew.