Thus we call folk liars/murderers when it matters.
In general, this seems to hit the nail on the head quite well, and a lot of things that other posters have said here would seem to be along the same lines. It is at least the best answer we have up to now, here in the thread. That is to say that labels are situation-dependent.
So I guess the next question might be, 'when does it matter?' or 'how do we know when it matters?' or better still, since 'it mattering' is going to be a matter of personal (and/or social) judgement, 'when do we
feel it matters (enough to be applicable to the whole person as a label after just one instance rather than waiting to see if its part of a predictable pattern)?
I have suggested that survival and/or reproductive situations might feature prominently. I'm not sure how well that holds up and would welcome counter-examples.
fast, you gave an example of saying 'that driver is a jerk'. That could be said to be a life-or-death scenario, especially if we are using the same road as them at the same time, or they have cut across us, etc. In a lot of cases, we do not know the other driver
at all, other than in their dangerous driving and yet have no hesitation in defining them with a one-word label. Because it matters. Were we to not apply the label (and for example allow that the driver did something completely out-of-character and was highly likely to drive well generally), this assumption/prediction could get us killed, by the driver, in that situation.
I should not be surprised to find that there is often an emotional (or similar heightened state of instinctive arousal) component to attaching such blanket labels, because....in some ways...it is..irrational to do so, because no one complex person can deserve to be encompassed by them.