Question

An eye for an eye (and a tooth for a tooth).
Prov. If someone hurts you, you should punish the offender by hurting him or her in the same way. (An ancient principle of justice going back to biblical times.)
  When they were children, the two brothers operated on the principle of an eye for an eye, so that if the older one hit the younger one, the younger one was entitled to hit him back just as hard.