an eye for an eye这句话挺好理解的，就是以眼还眼，以牙还牙，报复的意思。

【例句】
An eye for an eye makes the whole world blind.
以眼还眼会使全世界都看不见。

He has treated you badly and we shall give him an eye for an eye for what he has done.
他待你不好，我们会对他以眼还眼，以牙还牙。