Rude behavior is what it is. I think it it seems pretty obvious that the cashier was rude and that was the OP's experience.

That said, there can be any number of motives for people's behavior. That doesn't mean that the behavior wasn't wrong still.

People always told me when I was working retail that the customer is always right, etc. I mean, sure. But you still have to act like a civilized human being. That goes for both parties (customer and service person). But, practically speaking, it is more important for the worker to be civilized than the customer. That's just the way things work. It's obviously not right for a customer to act angrily towards a worker but it seems like in this situation, the customer wasn't angry at all and was not even provoking this cashier.