You know, the one that says: "Treat others as you would be treated." And apparently there's a Silver Rule (who knew?) that is the 'negative' form of the Golden Rule, that is: "do not treat people in ways that you would not care to be treated."
It's interesting to consider the fact that the saying: "an eye for an eye" also captures a kind of negative Golden Rule—that is: "as you have been done to, so too should you do."
However (despite its sweetness), revenge is not an ideal way to live one's life. It tends to start an endless cycle of retaliation—because your entirely appropriate and justified "revenge" is often seen by the revengee (?) as an act that needs to be—you guessed it—avenged! And so it goes, back & forth, escalating and unstoppable until there is no one or nothing left. Not so very sweet after all.
But there is a reverse angle view of the Rule: that if you do something, you are giving cosmic permission to have it done to you. Because by your action, you have declared it is an OK thing to do. So of course it is OK for others to do as well.
So if you, for example, cut in front of people in line, you really have no right to complain when others cut in front of you. You have set your seal of approval on the action. If you're dishing it out, you've got to take it—that's only fair.
It's something to think about when caught in a retaliatory action. Perhaps someone says something sexist and negative about women. It's almost automatic (if you're a woman) to say something sexist and negative back about men. Justified, as really, they are just being given a taste of their own medicine. Surely that will offer insight and learning!
But actually by that reaction, there is an implied declaration that being sexist is an OK thing to be. The two people simply disagree as to what justifies being sexist, or racist, or whatever. But being sexist or racist or whatever is clearly OK, because both parties are actively participating in being sexist (or racist, or whatever).
Another example is if people make negative statements about "rich people." They are, by definition, endorsing any parallel behavior that makes negative statements about "poor people." Because they clearly indicate it's entirely justifiable to make negative statements about "people" based on their financial status. Of course they happen to think it's only justified when people have more Vs less money, but really the concept is fine.
I loved Susan Elizabeth Phillips' Ain't She Sweet, but I always wanted Sugar to say that every one's retaliatory meanness comforted her, because her enemies had sunk to the same level, behaving in just as appalling a manner as she had. The issue was not that the behavior was bad and should not have happened—by their actions, that kind of behavior was fine. They just disagreed on what justified it.
It is not easy to quell the desire to retaliate. But if the behavior is wrong, it is wrong.