Two Wrongs Make a Right: “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.”
http://www.nizkor.org/features/fallacies/two-wrongs-make-a-right.html
In this fallacy, person A claims that since person B would do something to them, then it is acceptable for person A to do it as well even when it is not necessary in order to prevent person B from doing it to person A. “This sort of ‘reasoning’ is fallacious because an action that is wrong is wrong even if another person would also do it.” It is possible, however, to make this an okay situation if person A does X in order to defend themself from person B, such as an attack from person B. It could also be acceptable in the case of “justified retribution”, such as country A invading country B because country B plans to invade country A to enslave them – this would be called “launching a pre-emptive strike.” I think a common case of this fallacy in real life is borrowing money and lending money. For example, a person may borrow money from a friend to buy lunch but never pays their friend back in the belief that his friend would not pay him back anyway if he loaned money to his friend. It’s still wrong that the person borrowing does not pay back the money borrowed, despite claims and beliefs. Unfortunately, this happens around school very often.
Additionally, it could be too hasty of the person to claim that his friend would not pay him back either. There is no possible way of knowing until he has loaned money as well, nor is there anyway of knowing if even if he were not paid back that he would actually be paid back if he decided to pay back his friend first. It could actually end up serving as a gentle reminder to his friend that since he has been honest and paid back that he deserves to be paid back in kind. Another simple way, actually, is to just politely remind the friend. All of these possibilities support that the fallacy that not paying back a friend is justifiable is likely ridiculous.
http://www.bbc.co.uk/news/world-middle-east-13534775
That is an example of a real life fallacy, in which Palestine refuses to forgive Israel and accept a peace treaty set forth by the US.
There are other established examples, such as from the source linked in the beginning.
1. Bill has borrowed Jane’s expensive pen, but found he didn’t return it. He tell’s himself that it is okay to keep it, since she would have taken his.
2. Jane: “Did you hear about those terrorists killing those poor people? That sort of killing is just wrong.” Sue: “Those terrorists are justified. After all, their land was taken from them. It is morally right for them to do what they do.” Jane: “Even when they blow up busloads of children?” Sue: “Yes.”
3. After leaving a store, Jill notices that she has underpaid by $10. She decides not to return the money to the store because if she had overpaid, they would not have returned the money.
4. Jill is horrified by the way the state uses capital punishment. Bill says that capital punishment is fine, since those the state kill don’t have any qualms about killing others.
All these examples of the fallacy “Two Wrongs Make a Right” may seem reasonable at first, but after consideration are not. Number 1, for example, is similar to paying back. If Bill gives back the pen, could it not serve as a gentle reminder to Jane that Jane has his pen as well? Or number 4 – is killing a criminal any worse than what the criminal does? Sure the criminal may have done things, but now we are doing the same. We are not any better. Of course, this depends on the perspective of the person/judge. Some people simply believe in full retribution and that some criminals never change, thus supporting capital punishment. Yet others believe in life and rights, thus not supporting capital punishment. It is in the eye of the beholder to decide what is right and wrong in the end.