And anger indeed signifies a desire for some evil, that is, the harm one seeks to inflict on a neighbor, but anger desires that evil under the aspect of good, namely, righteous vengeance, not under the aspect of evil. For an angry person seeks to injure another in order to avenge an injury inflicted on the person....Therefore we should say regarding the question at issue that anger will be good and virtuous and called zealous if it be a desire for vengeance insofar as it is really righteous. But anger is a sin if it should belong to a vengeance apparently and falsely righteous. And Gregory in his work Morals calls such anger sinful.
Thomas Aquinas, De Malo 12.2. The Latin for 'vengeance' here is 'vindicatio'.
[Thomas Aquinas, On Evil, Regan, tr., Oxford University Press (Oxford: 2003) p. 378.]
