Objection 1. It would seem that there. is not a vice opposed to anger, resulting from lack of anger. For no vice makes us like to God. Now by being entirely without anger, a man becomes like to God, Who judges "with tranquillity" (Wisdom 12:18). Therefore seemingly it is not a vice to be altogether without anger.
Objection 2. Further, it is not a vice to lack what is altogether useless. But the movement of anger is useful for no purpose, as Seneca proves in the book he wrote on anger (De Ira i, 9, seqq.). Therefore it seems that lack of anger is not a vice.
Objection 3. Further, according to Dionysius (Div. Nom. iv), "man's evil is to be without reason." Now the judgment of reason remains unimpaired, if all movement of anger be done away. Therefore no lack of anger amounts to a vice.
On the contrary, Chrysostom [Hom. xi in Matth. in the Opus Imperfectum, falsely ascribed to St. John Chrysostom] says: "He who is not angry, whereas he has cause to be, sins. For unreasonable patience is the hotbed of many vices, it fosters negligence, and incites not only the wicked but even the good to do wrong."
I answer that, Anger may be understood in two ways. On one way, as a simple movement of the will, whereby one inflicts punishment, not through passion, but in virtue of a judgment of the reason: and thus without doubt lack of anger is a sin. This is the sense in which anger is taken in the saying of Chrysostom, for he says (Hom. xi in Matth., in the Opus Imperfectum, falsely ascribed to St. John Chrysostom): "Anger, when it has a cause, is not anger but judgment. For anger, properly speaking, denotes a movement of passion": and when a man is angry with reason, his anger is no longer from passion: wherefore he is said to judge, not to be angry. On another way anger is taken for a movement of the sensitive appetite, which is with passion resulting from a bodily transmutation. This movement is a necessary sequel, in man, to the movement of his will, since the lower appetite necessarily follows the movement of the higher appetite, unless there be an obstacle. Hence the movement of anger in the sensitive appetite cannot be lacking altogether, unless the movement of the will be altogether lacking or weak. Consequently lack of the passion of anger is also a vice, even as the lack of movement in the will directed to punishment by the judgment of reason.
Reply to Objection 1. He that is entirely without anger when he ought to be angry, imitates God as to lack of passion, but not as to God's punishing by judgment.
Reply to Objection 2. The passion of anger, like all other movements of the sensitive appetite, is useful, as being conducive to the more prompt execution [Cf. I-II, 24, 3 of reason's dictate: else, the sensitive appetite in man would be to no purpose, whereas "nature does nothing without purpose" [Aristotle, De Coelo i, 4.
Reply to Objection 3. When a man acts inordinately, the judgment of his reason is cause not only of the simple movement of the will but also of the passion in the sensitive appetite, as stated above. Wherefore just as the removal of the effect is a sign that the cause is removed, so the lack of anger is a sign that the judgment of reason is lacking.
Source: Summa Theologica (New Advent)