Can you explain something
The thing immitates you and looks and behaves just like you, so does that mean for instance, that if macready was "possessed" by the thing, he would still activiley hunt the other things and try to sabotage them?

In my opinion, yes. Having said that, the issue of the extent to which the behavior of a Thing imitation resembles that of the original creature comes up time and time again in various threads and contexts. Is it worth a sticky? XidiouX

I believe in the "slow infection theory". I believe that if you are slowly being infected that you could go for a while without knowing that you are infected. Like I think when Norris turns down leadership of the group, I think that he was suspicious that he might be infected. So yes, I do believe that Mac could be infected and hunt down the infected at the same time (What an interesting plot-point that would be if Mac gave himself the blood -*test*-('") and failed it!).