Cats absolutely know. Not in a moral "this is wrong" way like a person would feel guilt, but they know exactly what gets a reaction out of you. That stare down before the swat is the tell. Rufus wasn't confused, he was checking to see if you were watching. Your late breakfast probably had nothing to do with it though, the shelf thing was just more fun in the moment. They live in the now, so cause and effect only clicks for them when it comes to getting your attention, not because they feel bad about the cup.

Did you at least feel like a secret agent on a dead escalator?

Did you notice how the stress mostly came from holding in your reaction instead of the actual customer? When you stop treating every complaint like a test you have to pass, you stop carrying it around after your shift ends. The hard part is staying calm while you do it, but once you get that down the job gets a whole lot easier.