This fallacy involves attempting to justify doing something bad to a person or persons on the grounds that they would do it us. So, for example:
Bert regularly sells cars he knows are no good, so I'm justified in selling him this car that I know is no good. After all, he'd do the same to me.
Zenda would certainly nuke us, given the opportunity, so we are justified in nuking Zenda.
The truth, of course, is that two wrongs do not make a right. It may be true that a would do F to b; it doesn't follow that b is then justified in doing F to a.
Indeed, even if a has actually done F to b, b is still not necessarily justified in doing F to a (though perhaps, with additional premises, such a justification might be constructed: perhaps doing F in response might deter a from doing F again, or perhaps doing F to a might be justly deserved punishment for a's doing F to b).
Under some circumstances, the fact that someone has not yet done, but is clearly about to do, something bad to you may justify doing it to them first, but again, only with help of additional premises, as in this example: “Bert clearly is about to kill me, and the only way to stop him is to kill him first, so I am justified killing him in self defence”.
