In Ancient Israel's Criminal Law, Anthony Phillips argues that the death penalty in ancient Israel functioned neither for the purpose of strict retribution nor for deterrence, the standard grounds upon which it has been defended by Christians and still is in many circles. Rather, when a capital crime was committed, the covenant with Yahweh had been radically severed, and the whole community was liable to suffer God's wrath. The criminal therefore had to be executed to appease God's wrath and restore the covenant relationship. In other words, though Phillips does not quite use this terminology, the death penalty had almost a more cultic than judicial function; it was conceptually quite close to the sacrificial system.
If this is true, it offers a rather straightforward route for denying the death penalty's continuing relevance in the New Covenant. Christ has satisfied Yahweh's wrath once and for all and guarantees that the covenant will never be broken, that God's mercy will never depart from his people. No crime, then, can damage that relationship so as to invite God's wrath requiring the death of the offender. This argument does not, of course, demonstrate that the death penalty is necessarily wrong and wicked, only that there is no Christian basis for supporting it.