I was always under the belief that the Thing's were completely aware of the fact they weren't human anymore and would happily play each other off to keep a majority rule.
I.e - sacrificing one part of itself to keep the majority alive.
Though it does raise the (already mentioned) part of the transition stage, does a human know they are infected? Assuming they've been infected subtly rather than forcefully. A scary thought really.
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Another theory is that the Thing only turned violent when the dogs attacked, as prior to that point it was quite happy to just sit around and do nothing, which raises the point that the Thing is not fully destructive, just defensive. I like to believe the Dog Thing was more than happy to just survive for a bit. There's no real proof it assimilated anyone in the American camp prior to being discovered (?) - But i've not watched it for a while, so I may be wrong.
But yes, it's the best horror movie ever made.