This fallacy is committed when a person “justifies” an action X by claiming that the victim would do the same thing to the offender. A morally wrong action is still wrong even if another person/group would also do it.
It is claimed that person/group B would do action X to person/group A
It is acceptable for person/group A to do action X to person/group B (when A's doing X to B is not necessary to prevent B from doing X to A)
Alan: “Palestinian terrorism against Israeli civilians is wrong”
Noam: “Those terrorist actions are justified, because Israel has occupied them”
Alan: “Even when they kill innocent children?”
The argument is not fallacious if B’s action is necessary to prevent evil upon him. For example, if group A is planning terrorize group B, then group B would be justified in launching a pre-emptive strike to prevent the terrorism.