It's frowned upon to fantasize about violence.
Stan just had a bad argument with a customer, and he can't stop thinking about it.
Stan wants to think about what he'll do the next time it happens.
Stan thinks about what he'll say to diffuse the situation the next time.
Stan imagines grabbing the customer by the throat and throwing them against the wall.
Stan becomes better at dealing with heated arguments and will be better prepared.
Stan's thoughts start becoming more violent, and he's more willing to attack people.