Two Wrongs Make a Right is a fallacy in which a person "justifies" an action against a person by asserting that the person would do the same thing to him/her, when the action is not necessary to prevent B from doing X to A. This fallacy has the following pattern of "reasoning":
It is claimed that person B would do X to person A.
It is acceptable for person A to do X to person B (when A's doing X to B is not necessary to prevent B from doing X to A).
This sort of "reasoning" is fallacious because an action that is wrong is wrong even if another person would also do it.
It should be noted that it can be the case that it is not wrong for A to do X
to B if X is done to prevent B from doing X to A or if X is done in justified
retribution. For example, if Sally is running in the park and Biff tries to
attack her, Sally would be justified in attacking Biff to defend herself. As
another example, if country A is planning to invade country B in order to
enslave the people, then country B would be justified in launching a pre-emptive
strike to prevent the invasion.
Bill has borrowed Jane's expensive pen, but found he didn't return it. He tells himself that it is okay to keep it, since she would have taken his.Jane: "Did you hear about those terrorists killing those poor people? That sort of killing is just wrong."