I recently had the misfortune of having a crap customer reassigned to me from another representative at my company. I've learned firsthand why the customer was unsatisfied with the previous rep: this customer's a nasty piece of work and certainly would not inspire someone to go above and beyond, and perceives every unexpected turn as a slight against him by the service rep.
When I have courteous and polite people to deal with, I'll exhaust all resources to engineer a good outcome for them. I'll do my best at every turn, but when I'm badly treated, I'm certainly not going to open myself up to further opportunities to enjoy more abuse.
I really wonder what people think they are accomplishing when they threaten, bark orders and serve up heaping helpings of withering contempt to the people who assist them in service positions? Such people think they can intimidate other folks into doing a better job. Not so, I say.
These are precisely the same people who probably routinely send food back to the kitchens of restaurants they frequent. I am sure they are in part frustrated by the generally poor quality of service they encounter, because they merit no better. I guarantee you they have ingested more than a bit of someone else's bodily fluids. I mentioned this to one such person once, citing the likelihood there'd be extra additives in their replacement plate of hash, and they said, aghast, "they wouldn't dare." Guess what, sweetie? They totally dare. They were probably hugging themselves with glee as they peeked from the kitchen and watched you nom up their spittle. Or worse.