Taking It Easy

Working the lunch rush today in a grocery store, and let's just say I'm not feeling very patient. A customer comes up to the self-checkout and puts their items down like it's no big deal, then starts reading something on their phone while waiting for me. Now, here’s where I messed up, instead of being super professional as usual, I decided to make a comment under my breath about how rude people can be these days when all the self-checkouts are clearly there just to avoid dealing with actual employees. And you know what? As soon as they heard me, they shot back something equally not nice and stormed off. So here’s my question: Is it really that bad of a mistake for trying to vent in my head during a busy day?