Objection 1. It would seem that anger is not only towards those to whom one has an obligation of justice. For there is no justice between man and irrational beings. And yet sometimes one is angry with irrational beings; thus, out of anger, a writer throws away his pen, or a rider strikes his horse. Therefore anger is not only towards those to whom one has an obligation of justice.
Objection 2. Further, "there is no justice towards oneself... nor is there justice towards one's own" (Ethic. v, 6). But sometimes a man is angry with himself; for instance, a penitent, on account of his sin; hence it is written (Psalm 4:5): "Be ye angry and sin not." Therefore anger is not only towards those with whom one has a relation of justice.
Objection 3. Further, justice and injustice can be of one man towards an entire class, or a whole community: for instance, when the state injures an individual. But anger is not towards a class but only towards an individual, as the Philosopher states (Rhet. ii, 4). Therefore properly speaking, anger is not towards those with whom one is in relation of justice or injustice.
The contrary, however, may be gathered from the Philosopher (Rhet. ii, 2,3).
I answer that, As stated above (Article 6), anger desires evil as being a means of just vengeance. Consequently, anger is towards those to whom we are just or unjust: since vengeance is an act of justice, and wrong-doing is an act of injustice. Therefore both on the part of the cause, viz. the harm done by another, and on the part of the vengeance sought by the angry man, it is evident that anger concerns those to whom one is just or unjust.
Reply to Objection 1. As stated above (4, ad 2), anger, though it follows an act of reason, can nevertheless be in dumb animals that are devoid of reason, in so far as through their natural instinct they are moved by their imagination to something like rational action. Since then in man there is both reason and imagination, the movement of anger can be aroused in man in two ways. First, when only his imagination denounces the injury: and, in this way, man is aroused to a movement of anger even against irrational and inanimate beings, which movement is like that which occurs in animals against anything that injures them. Secondly, by the reason denouncing the injury: and thus, according to the Philosopher (Rhet. ii, 3), "it is impossible to be angry with insensible things, or with the dead": both because they feel no pain, which is, above all, what the angry man seeks in those with whom he is angry: and because there is no question of vengeance on them, since they can do us no harm.
Reply to Objection 2. As the Philosopher says (Ethic. v, 11), "metaphorically speaking there is a certain justice and injustice between a man and himself," in so far as the reason rules the irascible and concupiscible parts of the soul. And in this sense a man is said to be avenged on himself, and consequently, to be angry with himself. But properly, and in accordance with the nature of things, a man is never angry with himself.
Reply to Objection 3. The Philosopher (Rhet. ii, 4) assigns as one difference between hatred and anger, that "hatred may be felt towards a class, as we hate the entire class of thieves; whereas anger is directed only towards an individual." The reason is that hatred arises from our considering a quality as disagreeing with our disposition; and this may refer to a thing in general or in particular. Anger, on the other hand, ensues from someone having injured us by his action. Now all actions are the deeds of individuals: and consequently anger is always pointed at an individual. When the whole state hurts us, the whole state is reckoned as one individual [Cf. 29, 6].
Source: Summa Theologica (New Advent)