I think mean people are just stupid people who think that being mean, rude, and generally nasty somehow hides their stupidity. For example.
The jackass who makes my cashier cry by making some racist comment about her accent. Dude - this is America - we ALL have accents of one kind or another - if you don't like it, stay under the rock you came out from!
The lady who walks up and interrupts my conversation with another customer because she has to know where the Breathe Right strips are RIGHT NOW! (yo, beeyatch, you have one stuck to your nose already - you can wait a second, OK?) Then, she proceeds to call ME rude because I make her wait a second. WTF????
The woman who comes in shortly after our power comes back on (some bozo drove a car into a power pole an hour ago), proceeds to throw a fit because her script's not ready. We tell her that we were in the process of working on it when the power went out, and it got stuck in the system - we're fixing the problem now and it will be about 20 more minutes. She walks over to her companion and proceeds to go off about how all the people working here are stupid because we can't get her stuff ready in time for her to pick up. Let me guess - YOU'RE the idiot who hit the power pole???
Then - saving the best for last - the one who's mean to the wrong person. The gal who's friend was just arrested for altering a prescription for a controlled substance that decides it's a good idea to cuss out the sheriff... Oh yeah that will definitely get the deputy to unlock those cuffs!