Objection 1. It would seem that the object of anger is evil. For Gregory of Nyssa says [Nemesius, De Nat. Hom. xxi.] that anger is "the sword-bearer of desire," inasmuch, to wit, as it assails whatever obstacle stands in the way of desire. But an obstacle has the character of evil. Therefore anger regards evil as its object.
Objection 2. Further, anger and hatred agree in their effect, since each seeks to inflict harm on another. But hatred regards evil as its object, as stated above (Question 29, Article 1). Therefore anger does also.
Objection 3. Further, anger arises from sorrow; wherefore the Philosopher says (Ethic. viii, 6) that "anger acts with sorrow." But evil is the object of sorrow. Therefore it is also the object of anger.
On the contrary, Augustine says (Confess. ii, 6) that "anger craves for revenge." But the desire for revenge is a desire for something good: since revenge belongs to justice. Therefore the object of anger is good.
Moreover, anger is always accompanied by hope, wherefore it causes pleasure, as the Philosopher says (Rhet. ii, 2). But the object of hope and of pleasure is good. Therefore good is also the object of anger.
I answer that, The movement of the appetitive power follows an act of the apprehensive power. Now the apprehensive power apprehends a thing in two ways. First, by way of an incomplex object, as when we understand what a man is; secondly, by way of a complex object, as when we understand that whiteness is in a man. Consequently in each of these ways the appetitive power can tend to both good and evil: by way of a simple and incomplex object, when the appetite simply follows and adheres to good, or recoils from evil: and such movements are desire, hope, pleasure, sorrow, and so forth: by way of a complex object, as when the appetite is concerned with some good or evil being in, or being done to, another, either seeking this or recoiling from it. This is evident in the case of love and hatred: for we love someone, in so far as we wish some good to be in him; and we hate someone, in so far as we wish some evil to be in him. It is the same with anger; for when a man is angry, he wishes to be avenged on someone. Hence the movement of anger has a twofold tendency: viz. to vengeance itself, which it desires and hopes for as being a good, wherefore it takes pleasure in it; and to the person on whom it seeks vengeance, as to something contrary and hurtful, which bears the character of evil.
We must, however, observe a twofold difference in this respect, between anger on the one side, and hatred and love on the other. The first difference is that anger always regards two objects: whereas love and hatred sometimes regard but one object, as when a man is said to love wine or something of the kind, or to hate it. The second difference is, that both the objects of love are good: since the lover wishes good to someone, as to something agreeable to himself: while both the objects of hatred bear the character of evil: for the man who hates, wishes evil to someone, as to something disagreeable to him. Whereas anger regards one object under the aspect of evil, viz. the noxious person, on whom it seeks to be avenged. Consequently it is a passion somewhat made up of contrary passions.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)