eye for an eye and a tooth for a tooth
A blow or injury should be given back as hard as each one that is received; every crime or injury should be punished or paid back.
In ancient times if a man's eye was put out by his enemy, he might get revenge by putting his enemy's eye out. This was the rule of "an eye for an eye and a tooth for a tooth."
Sometimes used in a short form.
Churches today teach that we should forgive people who hurt us, not follow the rule of "an eye for an eye." (From the old command in the Bible meaning when you pay back a person, you should not hurt him more than he hurt you.)
Compare: DOG EAT DOG, GIVE AS GOOD AS ONE GETS, TIT FOR TAT.
Source: A Dictionary of American Idioms