On the contrary, Injustice is contrary to justice. But justice is a special virtue. Therefore injustice is a special vice.
I answer that, Injustice is twofold. First there is illegal injustice which is opposed to legal justice: and this is essentially a special vice, in so far as it regards a special object, namely the common good which it contemns; and yet it is a general vice, as regards the intention, since contempt of the common good may lead to all kinds of sin. Thus too all vices, as being repugnant to the common good, have the character of injustice, as though they arose from injustice, in accord with what has been said above about justice (Q. 58, AA. 5, 6). Secondly we speak of injustice in reference to an inequality between one person and another, when one man wishes to have more goods, riches for example, or honors, and less evils, such as toil and losses, and thus injustice has a special matter and is a particular vice opposed to particular justice.
Reply Obj. 1: Even as legal justice is referred to human common good, so Divine justice is referred to the Divine good, to which all sin is repugnant, and in this sense all sin is said to be iniquity.
Reply Obj. 2: Even particular justice is indirectly opposed to all the virtues; in so far, to wit, as even external acts pertain both to justice and to the other moral virtues, although in different ways as stated above (Q. 58, A. 9, ad 2).
Reply Obj. 3: The will, like the reason, extends to all moral matters, i.e. passions and those external operations that relate to another person. On the other hand justice perfects the will solely in the point of its extending to operations that relate to another: and the same applies to injustice. _______________________
SECOND ARTICLE [II-II, Q. 59, Art. 2]
Whether a Man Is Called Unjust Through Doing an Unjust Thing?
Objection 1: It would seem that a man is called unjust through doing an unjust thing. For habits are specified by their objects, as stated above (I-II, Q. 54, A. 2). Now the proper object of justice is the just, and the proper object of injustice is the unjust. Therefore a man should be called just through doing a just thing, and unjust through doing an unjust thing.
Obj. 2: Further, the Philosopher declares (Ethic. v, 9) that they hold a false opinion who maintain that it is in a man's power to do suddenly an unjust thing, and that a just man is no less capable of doing what is unjust than an unjust man. But this opinion would not be false unless it were proper to the unjust man to do what is unjust. Therefore a man is to be deemed unjust from the fact that he does an unjust thing.
Obj. 3: Further, every virtue bears the same relation to its proper act, and the same applies to the contrary vices. But whoever does what is intemperate, is said to be intemperate. Therefore whoever does an unjust thing, is said to be unjust.