Objection 1. It would seem that anger is the most grievous sin. For Chrysostom says [Hom. xlviii in Joan.] that "nothing is more repulsive than the look of an angry man, and nothing uglier than a ruthless* face, and most of all than a cruel soul." ['Severo'. The correct text is 'Si vero.' The translation would then run thus... 'and nothing uglier.' And if his 'face is ugly, how much uglier is his soul!']. Therefore anger is the most grievous sin.
Objection 2. Further, the more hurtful a sin is, the worse it would seem to be; since, according to Augustine (Enchiridion xii), "a thing is said to be evil because it hurts." Now anger is most hurtful, because it deprives man of his reason, whereby he is master of himself; for Chrysostom says (Hom. xlviii in Joan.) that "anger differs in no way from madness; it is a demon while it lasts, indeed more troublesome than one harassed by a demon." Therefore anger is the most grievous sin.
Objection 3. Further, inward movements are judged according to their outward effects. Now the effect of anger is murder, which is a most grievous sin. Therefore anger is a most grievous sin.
On the contrary, Anger is compared to hatred as the mote to the beam; for Augustine says in his Rule (Ep. ccxi): "Lest anger grow into hatred and a mote become a beam." Therefore anger is not the most grievous sin.
I answer that, As stated above (1 and 2), the inordinateness of anger is considered in a twofold respect, namely with regard to an undue object, and with regard to an undue mode of being angry. As to the appetible object which it desires, anger would seem to be the least of sins, for anger desires the evil of punishment for some person, under the aspect of a good that is vengeance. Hence on the part of the evil which it desires the sin of anger agrees with those sins which desire the evil of our neighbor, such as envy and hatred; but while hatred desires absolutely another's evil as such, and the envious man desires another's evil through desire of his own glory, the angry man desires another's evil under the aspect of just revenge. Wherefore it is evident that hatred is more grievous than envy, and envy than anger: since it is worse to desire evil as an evil, than as a good; and to desire evil as an external good such as honor or glory, than under the aspect of the rectitude of justice. On the part of the good, under the aspect of which the angry man desires an evil, anger concurs with the sin of concupiscence that tends to a good. On this respect again, absolutely speaking. the sin of anger is apparently less grievous than that of concupiscence, according as the good of justice, which the angry man desires, is better than the pleasurable or useful good which is desired by the subject of concupiscence. Wherefore the Philosopher says (Ethic. vii, 4) that "the incontinent in desire is more disgraceful than the incontinent in anger."
On the other hand, as to the inordinateness which regards the mode of being angry, anger would seem to have a certain pre-eminence on account of the strength and quickness of its movement, according to Proverbs 27:4, "Anger hath no mercy, nor fury when it breaketh forth: and who can bear the violence of one provoked?" Hence Gregory says (Moral. v, 45): "The heart goaded by the pricks of anger is convulsed, the body trembles, the tongue entangles itself, the face is inflamed, the eyes are enraged and fail utterly to recognize those whom we know: the tongue makes sounds indeed, but there is no sense in its utterance."
Reply to Objection 1. Chrysostom is alluding to the repulsiveness of the outward gestures which result from the impetuousness of anger.
Reply to Objection 2. This argument considers the inordinate movement of anger, that results from its impetuousness, as stated above.
Reply to Objection 3. Murder results from hatred and envy no less than from anger: yet anger is less grievous, inasmuch as it considers the aspect of justice, as stated above.
Source: Summa Theologica (New Advent)