Deuteronomy 19:13

19:13 You must not pity him, but purge out the blood of the innocent ¹  from Israel, so that it may go well with you.

Deuteronomy 19:21

19:21 You must not show pity; the principle will be a life for a life, an eye for an eye, a tooth for a tooth, a hand for a hand, and a foot for a foot. ² 

[19:13]  ¹ sn Purge out the blood of the innocent. Because of the corporate nature of Israel’s community life, the whole community shared in the guilt of unavenged murder unless and until vengeance occurred. Only this would restore spiritual and moral equilibrium (Num 35:33).

[19:21]  ² sn This kind of justice is commonly called lex talionis or “measure for measure” (cf. Exod 21:23-25; Lev 24:19-20). It is likely that it is the principle that is important and not always a strict application. That is, the punishment should fit the crime and it may do so by the payment of fines or other suitable and equitable compensation (cf. Exod 22:21; Num 35:31). See T. S. Frymer-Kensky, “Tit for Tat: The Principle of Equal Retribution in Near Eastern and Biblical Law,” BA 43 (1980): 230-34.