4. Good stands in no need of evil, but the other way about (Chap. XI). Whatever then is of necessity for the preservation of good, cannot be of itself evil.* But for the preservation of concord among men it is necessary for penalties to be inflicted on the wicked.
5. The common good is better than the good of the individual. There fore some particular good must be withdrawn for the preservation of the common good. But the life of certain pestilent fellows is a hindrance to the common good, that is, to the concord of human society. Such persons therefore are to be withdrawn by death from the society of men.*
Hence the Apostle says: He beareth not the sword in vain (Rom. xiii, 4: cf. 1 Pet. ii, 14).
Hereby is excluded the error of those who say that corporal punishments are unlawful, and quote in support of their error such texts as, Thou shalt not kill (Exod. xx, 13): Let both grow until the harvest (Matt. xiii, 30). But these are frivolous allegations. For the same law which says, Thou shalt not kill, adds afterwards: Thou shalt not suffer poisoners (maleficos, pharmakous) to live (Exod. xxii, 18). And as for both growing until the harvest, how that is to be understood appears from what follows: lest perchance in gathering the tares ye root out along with them the wheat also: in this passage then the killing of the wicked is forbidden where it cannot be done without danger to the good, as happens when the wicked are not yet clearly marked off from the good by manifest sins, or when there is ground for apprehension that the wicked may involve many good men in their ruin.
The fate of the wicked being open to conversion so long as they live does not preclude their being open also to the just punishment of death. Indeed the danger threatening the community from their life is greater and more certain than the good expected by their conversion. Besides, in the hour of death, they have every facility for turning to God by repentance. And if they are so obstinate that even in the hour of death their heart will not go back upon its wickedness, a fairly probable reckoning may be made that they never would have returned to a better mind.
3.146 : That Sins are punished also by the experience of something Painful
3.148 : That Man stands in need of Divine Grace for the Gaining of Happiness