On topic, I find a lot (not all mind you because I have my stories too) "bad cop" stories involve someone breaking a law (albeit usually a small one) and not getting away with it.
News flash, a cop that lets you off for speeding, rolling stops, illegal turns etc.. is being NICE!! and letting you get away with BREAKING THE LAW. A cop that tickets you for the same infraction is DOING HIS JOB!!
When it comes to an officers attitude, I always like to remember this (and it was mentioned earlier). An officer never knows when this traffic stop is going to lead to someone trying to shoot/attack them. This happens, and in big cities is more common. I can see being abrupt and harsh with someone when you have no idea if they might snap or not.