Very few people get that far. I think there's only one person whom I would call a "cunt" genuinely rather than endearingly, and that's my former boss. He was rude to colleagues, both below and above him, to friends, to waiters, to his girlfriend, and even to clients. I hated the two years I spent working for him, and there is nothing he could do now to change my opinion of him. Thankfully I haven't spoken to him in almost a year, and most likely never will again.
One person I used to be enemies with - for a few years we hated each other and were horrible towards each other whenever our paths cross (in my head he was at fault entirely, and I know he feels the same way) now works for me full time. Because of his talent I overlooked our relationship and gave him the break he needed, and in the 18 months since then we've had a great working relationship. Now I wouldn't say a bad thing about him.
The above is a drawn out way of saying: it is possible, if perhaps not common, to label people without it having negative consequences.
Actually, re-reading, your words were that if you label someone negatively, you're more likely to treat them badly. That may be the case, but for me, I'm no more likely to be rude to someone I label a jerk as someone I don't label a jerk, but that I notice acting like a jerk. The label isn't something I decide to add because of something, it's a connection my brain makes automatically - unless there is evidence that someone is not a jerk, there isn't a difference between them acting like a jerk and being a jerk.
(Written from my phone in bed so won't re-read what I just wrote, hopefully it makes at least a little sense.)
My point is that it is not helpful to think of people as "jerks". I actually think it can promote rude behavior to label people "jerks" because people may justify to themselves that it's okay to be rude to a "jerk".