Objection 1. It would seem that not everyone who does an injustice sins mortally. For venial sin is opposed to mortal sin. Now it is sometimes a venial sin to do an injury: for the Philosopher says (Ethic. v, 8) in reference to those who act unjustly: "Whatever they do not merely in ignorance but through ignorance is a venial matter." Therefore not everyone that does an injustice sins mortally.
Objection 2. Further, he who does an injustice in a small matter, departs but slightly from the mean. Now this seems to be insignificant and should be accounted among the least of evils, as the Philosopher declares (Ethic. ii, 9). Therefore not everyone that does an injustice sins mortally.
Objection 3. Further, charity is the "mother of all the virtues" [Peter Lombard, Sent. iii, D. 23, and it is through being contrary thereto that a sin is called mortal. But not all the sins contrary to the other virtues are mortal. Therefore neither is it always a mortal sin to do an injustice.
On the contrary, Whatever is contrary to the law of God is a mortal sin. Now whoever does an injustice does that which is contrary to the law of God, since it amounts either to theft, or to adultery, or to murder, or to something of the kind, as will be shown further on (64, seqq.). Therefore whoever does an injustice sins mortally.
I answer that, As stated above (I-II, 12, 5), when we were treating of the distinction of sins, a mortal sin is one that is contrary to charity which gives life to the soul. 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.
Reply to Objection 1. This saying of the Philosopher is to be understood as referring to ignorance of fact, which he calls "ignorance of particular circumstances" [Ethic. iii, 1, and which deserves pardon, and not to ignorance of the law which does not excuse: and he who does an injustice through ignorance, does no injustice except accidentally, as stated above (Article 2)
Reply to Objection 2. He who does an injustice in small matters falls short of the perfection on an unjust deed, in so far as what he does may be deemed not altogether contrary to the will of the person who suffers therefrom: for instance, if a man take an apple or some such thing from another man, in which case it is probable that the latter is not hurt or displeased.
Reply to Objection 3. The sins which are contrary to the other virtues are not always hurtful to another person, but imply a disorder affecting human passions; hence there is no comparison.
Source: Summa Theologica (New Advent)