I always saw the noose as Blair kind of saying 'fuck you' to everyone who locked him up, like saying 'you left me rope, I can make a noose, not killing myself because I'm not losing my mind'. It's there to make a point, however clumsily.

In the commentary, which is well worth a listen, Carpenter muses on identity during an earlier scene with Norris, where Garry offers him the gun and he doesn't want it. Carpenter wondered if Norris felt something was wrong with him and didn't trust himself with the responsibility for some reason, which also led to thoughts on if you're so perfectly copied by the thing, do you even know you're not you?

This in turn feeds into Palmer's response to the blood test He's completely himself tying the bodies up (they're dead Mac!), but once that hot wire comes to his petri dish, he has a sudden look of complete resignation. Not fear, not threat, not panic at his imminent discovery, but resignation, like 'oh... well, that's that then'.

Which then leads to the brilliant ending and if the survivors even know if they are or are not copies.

Also in the commentary, Carpenter talks about bad test screenings for the ambiguous finale, so they filmed an ending where Mac is rescued and has a blood test revealing he's not the thing. Would love to see it.