With regard to certain energy-sapping revenge fantasies floating around…
Have you ever interacted with a chronic cheater? Surely you have, on the playground or among your co-workers. In your experience, did imitating their behavior “show them” or make them feel any kind of shame? Or did it confirm to them that cheating is what anybody would do in this situation, and they were right to do it?
I can tell you, if you don’t have personal experience to draw on, there is a ton of social science research which says yes this second thing is exactly what happens.
If you are dealing with a cheater, the best you can manage is to minimize their damage to you, to the extent possible, until they can be excluded from the process. When you start cheating as well, all that does is signal to third parties that you agree with the cheater’s interpretation of what’s acceptable. Which makes it not cheating. Cheater wins! Cheater gets to stay.
If you have a contrasting experience, where “a taste of their own medicine” caused a cheater to reform, I’m interested. Sounds like a fun story to hear. But I bet you don’t have one.