Halakhah zu Dewarim 19:13

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."