I'm so pleased that Miss Manners agrees with me. Granted, her question wasn't about pie, but about a rude cashier. Her Advice: (may require registration--sorry!)
This is not to match or outdo the offender with a counterattack, but to report the incident to the manager. It would have relieved your indignation, and a conscientious manager would want to know why the business is losing a customer. And you should leave your speculations about mental health out of it.
So there, gentle readers, let management know.