an eye for an eye
Fixed expression from the biblical principle of retributive justice (Matthew 5:38). Often used in the fuller form 目には目を歯には歯を.
See also: 目には目を歯には歯を (めにはめをはにははを)
The idea of 'an eye for an eye' is not widely adopted in modern legal systems.
He acted with a spirit of 'an eye for an eye' for revenge.