Thomas Aquinas

“ Now every injury inflicted on another person is of itself contrary to charity, which moves us to will the good of another. And so since injustice always consists in an injury inflicted on another person, it is evident that to do an injustice is a mortal sin according to its genus. ”

“ In the latter case properly speaking it arises from a habit, because whenever a man has a habit, whatever befits that habit is, of itself, pleasant to him. Accordingly, to do what is unjust intentionally and by choice is proper to the unjust man, in which sense the unjust man is one who has the habit of injustice: but a man may do what is unjust, unintentionally or through passion, without having the habit of injustice. ”

“ 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. ”

“ To this object the habit of injustice is compared by means of its proper act which is called an injustice. Accordingly it may happen in two ways that a man who does an unjust thing, is not unjust: first, on account of a lack of correspondence between the operation and its proper object. ”

“ 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.
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. ”