Lord commanded them to forbear from uprooting the
cockle in order to spare the wheat, i.e. the good. This occurs when
the wicked cannot be slain without the good being killed with them,
either because the wicked lie hidden among the good, or because they
have many followers, so that they cannot be killed without danger to
the good, as Augustine says (Contra Parmen. iii, 2). Wherefore our
Lord teaches that we should rather allow the wicked to live, and that
vengeance is to be delayed until the last judgment, rather than that
the good be put to death together with the wicked. When, however, the
good incur no danger, but rather are protected and saved by the
slaying of the wicked, then the latter may be lawfully put to death.

Reply Obj. 2: According to the order of His wisdom, God sometimes
slays sinners forthwith in order to deliver the good, whereas
sometimes He allows them time to repent, according as He knows what
is expedient for His elect. This also does human justice imitate
according to its powers; for it puts to death those who are dangerous
to others, while it allows time for repentance to those who sin
without grievously harming others.

Reply Obj. 3: By sinning man departs from the order of reason, and
consequently falls away from the dignity of his manhood, in so far as
he is naturally free, and exists for himself, and he falls into the
slavish state of the beasts, by being disposed of according as he is
useful to others. This is expressed in Ps. 48:21: "Man, when he was
in honor, did not understand; he hath been compared to senseless
beasts, and made like to them," and Prov. 11:29: "The fool shall
serve the wise." Hence, although it be evil in itself to kill a man
so long as he preserve his dignity, yet it may be good to kill a man
who has sinned, even as it is to kill a beast. For a bad man is worse
than a beast, and is more harmful, as the Philosopher states (Polit.
i, 1 and _Ethic._ vii, 6).
_______________________

THIRD ARTICLE [II-II, Q. 64, Art. 3]

Whether It Is Lawful for a Private Individual to Kill a Man Who Has
Sinned?

Objection 1: It would seem lawful for a private individual to kill a
man who has sinned. For nothing unlawful is commanded in the Divine
law. Yet, on account of the sin of the molten calf, Moses commanded
(Ex. 32:27): "Let every man kill his brother, and friend, and
neighbor." Therefore it is lawful for private individuals to kill a
sinner.

Obj. 2: Further, as stated ab