Sefer HaChinuch ספר החינוך

To not show pity upon the killer or injurer: That we were prevented from having mercy upon the one that killed his fellow or removed one of his limbs; that the judge not say, “This poor man that cut off the hand of his fellow or blinded his eye, he did not do it intentionally,” and he have mercy upon him from having him repay [the injured] according to his evil. And about this is it stated (Deuteronomy 19:21), “And your eye shall not show mercy, a soul for a soul, etc.” And the prevention of this is repeated in a different place, as it is stated (Deuteronomy 19:13), “Your eye shall not show mercy upon him, and you shall purge the innocent blood.”

The root of this commandment is well-known — that if we do not punish the injurers and purge the evil from among us, “a man will swallow his neighbor alive”; and states will not be [properly] civilized. There is no need to speak at length about it.

The laws of the commandment are short [and] elucidated in the language of Scripture.

And this prohibition is practiced at the time of the [Temple] by males, as it is upon them to administer justice. And the judge that transgresses this and removes his eye from punishing the guilty one according to his evil has transgressed this negative commandment — but there are no lashes for it, since no act is [involved] with it — and his punishment is very great for the reason that we mentioned: that there is destruction to the civilization of the world with this thing. And even outside of the Land, even though we do not have the ability to judge capital cases, every court is obligated to punish their guilty ones according to their evil through that which they [are able] — whether monetarily or, if they can, corporeally — according to what they see that the time requires. For if there is no “rod always outstretched over the back of the fools,” the preservation of the people is impossible.