Objection 1. It would seem that injustice is not a special vice. For it is written (1 John 3:4): "All sin is iniquity [Vulgate: 'Whosoever committeth sin, committeth also iniquity; and sin is iniquity']." Now iniquity would seem to be the same as injustice, because justice is a kind of equality, so that injustice is apparently the same as inequality or iniquity. Therefore injustice is not a special sin.
Objection 2. Further, no special sin is contrary to all the virtues. But injustice is contrary to all the virtues: for as regards adultery it is opposed to chastity, as regards murder it is opposed to meekness, and in like manner as regards the other sins. Therefore injustice is not a special sin.
Objection 3. Further, injustice is opposed to justice which is in the will.
But every sin is in the will, as Augustine declares (De Duabus Anim. x). Therefore injustice is not a special sin.
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 (58, 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 to Objection 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 to Objection 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 (58, 09, ad 2).
Reply to Objection 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.
Source: Summa Theologica (New Advent)