"He hit me first." What parent has not heard that response when breaking up a fight amongst their kids. We seem to have the built-in idea that when someone does something to us, it immediately justifies a response that is at least equal, and often greater.
It is not always children who behave that way. We are seeing more and more adults resorting to the same kind of logic for many disproportionate responses.
There is help and answers for us when we find ourselves in the middle of these exchanges.