Do you ever get the urge when serving customers to..?

Customers are money on legs - that's why the business says they are always right. Staff are there to pander to the horrible ones so that they spend money, which gives the firm good profits and thus increases your wages. Sadly, a lot of people are totally rude and unpleasant. This is their loss, not yours and I'm not surprised you have evil thoughts about them. It's human. Just be ultra polite to such people. There are nonaggressive ways of getting your own back.....by going very S l o w l y or making them wait for you to serve them. Ultimately this will wind them up and it's their blood pressure that'll be rising....not yours.