To take someone’s life is the greatest of all thefts, for in stealing this one thing, everything else is stolen along with it. There are, in specific circumstances, justifiable killings, but these are carried out by a legitimate authority. The reason this is licit is due to the hierarchical nature of social authority. For a plant to be sacrificed to the animal, and the animal to the man, is not out of the normal hierarchy of life. However, two men who are hierarchically each other’s peers have no right to take such action. Thus, when it comes to the just extinction of a human life, we must turn to a superior authority, which in this case is the political authority. A properly constituted political authority is superior to the individual members of the social body from the standpoint of justice, and so the decision that one life must be removed from the whole can be justly made by that authority alone. Although the above reasoning legitimates the death penalty in theory, it must be remembered that even when a legitimate social authority exists, due to the intrinsic value of human life the death penalty should only be employed in cases of necessity, when society has no other means to protect itself from the menace of the criminal:
It is clear that, for the [purposes of punishment] to be achieved, the nature and extent of the punishment must be carefully evaluated and decided upon, and [the state] ought not go to the extreme of executing the offender except in cases of absolute necessity: in other words, when it would not be possible otherwise to defend society. Today however, as a result of steady improvements in the organization of the penal system, such cases are very rare, if not practically non-existent.
 ST II-II, q. 64, aa. 2-3; ST II-II, q. 65, a. 1.
 EV, 56. This passage also cited in the Catechism.