Adam Clarke on Deuteronomy 19:21

Life - for life, eye for eye, etc. - The operation of such a law as this must have been very salutary: if a man prized his own members, he would naturally avoid injuring those of others. It is a pity that this law were not still in force: it would certainly prevent many of those savage acts which now both disgrace and injure society. I speak this in reference to law generally, and the provision that should be made to prevent and punish ferocious and malevolent offenses. A Christian may always act on the plan of forgiving injuries; and where the public peace and safety may not be affected, he should do so; but if law did not make a provision for the safety of the community by enactment against the profligate, civil society would soon be destroyed.