Mac's Major Mistake? (spoilers obv.)

Watched this the other day for maybe the 8th or 10th time or so. One thing I noticed that really stuck out was that Mac made a huge error.

So, it wasn't easy, and it took them a while and a lot of blood and death, but using Mac's heated blood test, they actually were able to sniff out who was infected, who wasn't, and after taking out the Palmer-thing, and unfortunately losing Windows, ended up securing the team and knowing they were ok. Except for Blair... while they were doing all of that, the Blair-thing was having a field day not being watched.

The error that Mac made is that previously, when Mac spoke to Blair, he had pretty much 100% clear evidence that Blair had turned. I'm not sure why it was written this way, but we see the noose and know more or less what that means, but for some reason the characters don't react to the noose; they behave as if it isn't even there. The noose and Blair's behaviour pretty much immediately tell us that Blair has turned, where human Blair must have been about to kill himself just before turning. Now Blair-thing doesn't really understand what a noose is or that it's relevant (maybe), and just repeats "I'm ok now, I won't hurt anyone, please let me out" over and over like a robot.

If Mac had realized the significance of the noose (no idea why the characters just ignore it completely), then he could have taken out Blair-thing a lot sooner, which was the only thing left to do after taking out Palmer-thing.

(Of course, if he had, then things would have played out differently, and who knows where things would have ended up, but it just stuck out to me as Blair being the one detail he didn't cover, yet he had a giant neon-sign noose flashing in his face for a good 30+ seconds telling him to do something about Blair, but he completely missed it for some reason.)