I'm going to put nasty rinse water in a rude customer's coffee
No way. Why would you something so disgusting to a customer?
I just quit, and I want to make those rude customers suffer before I head out the door.
I understand that you want to lash out due to your mistreatment. However, doing something like that could land you in jail if caught.
I don't care about the law.
I understand you are frustrated, but I believe you shouldn't let your frustrations take a hold of your professional career. How would that customer feel knowing someone tampered with their drink, it's unsanitary and can cause them harm. How about just having a nice rant with your coworkers to let off steam?