And I certainly wouldn't claim to abide to a higher than average moral standard, I've been rude to my friends and to people I dislike. But not to my colleagues, not to strangers and not to people working indirectly for me (i.e. waiters etc.)
It is quite possible that in the future I might be rude to a waiter, but that would be the exception, not the rule. So yes, it's possible that if you judge someone of a single incident you might have caught the one in a thousand occurrence, chances are against it, far more likely is that you're seeing their normal behaviour.
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".
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.)
In other words, I find it more helpful to call out specific behavior as inappropriate than to write off the whole person as a "jerk".