I know Mac is the main character and hero of this film and therefore the film showed the Thing at its most large and threatening in a sort of showdown with him, but was there actually any reason for it to behave like that at the end, other than it just being the climax to the film? I mean it could have pretended to be one of the guys Mac was with and taken him out that way.
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It didn't need to pretend any longer. As far as it knew, Mac was the only human left, so why not "go large" and perhaps intimidate Mac into surrendering or at least hesitating.
Now that you point it out, I think it's actually entirely possible that The Thing was literally saying, "Fuck you" to Mac, crowing about its triumph before devouring him without pretense
The other character (I forget his name..Charles?) was still around, unless we're to believe he has been taken over already? In fact if we are to go with what you say about the Thing going large and intimidating because Mac is the last one, then it helps to answer the question of Charles being the Thing or not and the answer would be that he isn't.
Seemed like its chances of winning that particular battle though would have been improved if it had taken the quiet route of imitation of a colleague, rather than the "here I am!" final boss move.