John Gill Commentary Deuteronomy 19:21

"And thine eyes shall not pity; life [shall go] for life, eye for eye, tooth for tooth, hand for hand, foot for foot." — Deuteronomy 19:21 (ASV)

And your eye shall not pity
The false witness when convicted; this is directed to the judges, who should not spare such an one through favour or affection, but pronounce a righteous sentence on him, and see it executed, in proportion to the crime, and that according to the law of retaliation:

but life shall go for life ;
in such a case where the life of a person must have gone, if the falsehood of the testimony had not been discovered, the false witness must suffer death; in other cases, where a member would have been lost, or the price of it paid for, the same penalty was to be inflicted:

eye for eye, tooth for tooth, hand for hand, foot for foot ;
that is, the price of an eye an eye see (Exodus 21:23Exodus 21:24) (Leviticus 24:19Leviticus 24:20) .