Objection 1. It would seem that it is not essential to virtue that it should be a good habit. For sin is always taken in a bad sense. But there is a virtue even of sin; according to 1 Corinthians 15:56: "The virtue [Douay: 'strength'] of sin is the Law." Therefore virtue is not always a good habit.
Objection 2. Further, Virtue corresponds to power. But power is not only referred to good, but also to evil: according to Isaiah 5: "Woe to you that are mighty to drink wine, and stout men at drunkenness." Therefore virtue also is referred to good and evil.
Objection 3. Further, according to the Apostle (2 Corinthians 12:9): "Virtue [Douay: 'power'] is made perfect in infirmity." But infirmity is an evil. Therefore virtue is referred not only to good, but also to evil.
On the contrary, Augustine says (De Moribus Eccl. vi): "No one can doubt that virtue makes the soul exceeding good": and the Philosopher says (Ethic. ii, 6): "Virtue is that which makes its possessor good, and his work good likewise."
I answer that, As we have said above (Article 1), virtue implies a perfection of power: wherefore the virtue of a thing is fixed by the limit of its power (De Coelo i). Now the limit of any power must needs be good: for all evil implies defect; wherefore Dionysius says (Div. Hom. ii) that every evil is a weakness. And for this reason the virtue of a thing must be regarded in reference to good. Therefore human virtue which is an operative habit, is a good habit, productive of good works.
Reply to Objection 1. Just as bad things are said metaphorically to be perfect, so are they said to be good: for we speak of a perfect thief or robber; and of a good thief or robber, as the Philosopher explains (Metaph. v, text. 21). In this way therefore virtue is applied to evil things: so that the "virtue" of sin is said to be law, in so far as occasionally sin is aggravated through the law, so as to attain to the limit of its possibility.
Reply to Objection 2. The evil of drunkenness and excessive drink, consists in a falling away from the order of reason. Now it happens that, together with this falling away from reason, some lower power is perfect in reference to that which belongs to its own kind, even in direct opposition to reason, or with some falling away therefrom. But the perfection of that power, since it is compatible with a falling away from reason, cannot be called a human virtue.
Reply to Objection 3. Reason is shown to be so much the more perfect, according as it is able to overcome or endure more easily the weakness of the body and of the lower powers. And therefore human virtue, which is attributed to reason, is said to be "made perfect in infirmity," not of the reason indeed, but of the body and of the lower powers.
Source: Summa Theologica (New Advent)