St. Thomas is something of a champion of human reason, which makes his answer to the following objection all the more interesting:
...according to Dionysius, "The soul's evil is to be without reason." Now anger is always without reason: for the Philosopher says that "anger does not listen perfectly to reason"; and Gregory says that "when anger sunders the tranquil surface of the soul, it mangles and rends it by its riot"; and Cassian says: "From whatever cause it arises, the angry passion boils over and blinds the eye of the mind." Therefore it is always evil to be angry.
You can see the strength this argument might have against someone who believes the image of God is in man only through his reason.
Here is St. Thomas's reply, with my glosses:
Anger may stand in a twofold relation to reason. First, antecedently; in this way it withdraws reason from its rectitude, and has therefore the character of evil.
Here, St. Thomas grants the truth of the objection -- and remember, in his treatment anger is a vice opposed to temperance, so it's not like this article denies anger can be evil, or even that it usually is -- by making a distinction in anger's relation to reason. A person can get angry before reasoning the circs. through, which is wrong ("has the character of evil").
Secondly, consequently, inasmuch as the movement of the sensitive appetite is directed against vice and in accordance with reason, this anger is good, and is called "zealous anger."
A person can also get angry after or as a consequence of thinking things through, and if this anger is directed at correcting vice, and accords with reason, it's good.
In other words, St. Thomas is saying that it is possible to be angry in accord with reason, in which case the argument that "anger is always without reason" loses its strength.
So, is it possible to be angry in accord with reason?
Wherefore Gregory says: "We must beware lest, when we use anger as an instrument of virtue, it overrule the mind, and go before it as its mistress, instead of following in reason's train, ever ready, as its handmaid, to obey." This latter anger, although it hinder somewhat the judgment of reason in the execution of the act, does not destroy the rectitude of reason. Hence Gregory says that "zealous anger troubles the eye of reason, whereas sinful anger blinds it."
First, note that he quotes the same St. Gregory the Great who was quoted in the objection saying that anger rends and mangles the soul. So the distinction between sinful anger and zealous anger goes back well before St. Thomas.
What St. Thomas has in mind is the case where someone observes some injustice or vice, determines that it can be corrected, settles on a means to correct it, then becomes angry -- anger being a "movement of the sensitive appetite" associated with arduous desires. Anger is, so to speak, what fuels the flesh to help the spirit attain its end of avenging vice.
It's true, St. Thomas admits, that even this zealous anger "troubles the eye of reason. What he denies is that it is contrary or opposed to reason. The reasoning is, in a sense, already done; the anger is directed at carrying out reason's plan.
Nor is it incompatible with virtue that the deliberation of reason be interrupted in the execution of what reason has deliberated: since art also would be hindered in its act, if it were to deliberate about what has to be done, while having to act.
I find this a fascinating comparison. For St. Thomas, art is right reasoning about a thing to be made. Art is a type of reason, yet when it acts it doesn't reason. It's the whole process that is governed by reason -- hence human, hence virtuous -- not each individual component of the process. In fact, to insist that each individual component of the process be interrupted by the deliberation of reason is to hinder the overall process, if not to wreck it altogether. There are times when reasoning is unreasonable.
If this is true of art, then it can in principle be true of other things. Of anger, for instance. To determine whether it's true of art, I suspect we're better off asking artists rather than relying purely on philosophy. Similarly, we may be better off asking the virtuous whether they can be angry without sinning than trying to resolve that question with a purely speculative argument.