Objection 1. It would seem that vice, i.e. a bad habit, is worse than a sin, i.e. a bad act. For, as the more lasting a good is, the better it is, so the longer an evil lasts, the worse it is. Now a vicious habit is more lasting than vicious acts, that pass forthwith. Therefore a vicious habit is worse than a vicious act.
Objection 2. Further, several evils are more to be shunned than one. But a bad habit is virtually the cause of many bad acts. Therefore a vicious habit is worse than a vicious act.
Objection 3. Further, a cause is more potent than its effect. But a habit produces its actions both as to their goodness and as to their badness. Therefore a habit is more potent than its act, both in goodness and in badness.
On the contrary, A man is justly punished for a vicious act; but not for a vicious habit, so long as no act ensues. Therefore a vicious action is worse than a vicious habit.
I answer that, A habit stands midway between power and act. Now it is evident that both in good and in evil, act precedes power, as stated in Metaph. ix, 19. For it is better to do well than to be able to do well, and in like manner, it is more blameworthy to do evil, than to be able to do evil: whence it also follows that both in goodness and in badness, habit stands midway between power and act, so that, to wit, even as a good or evil habit stands above the corresponding power in goodness or in badness, so does it stand below the corresponding act. This is also made clear from the fact that a habit is not called good or bad, save in so far as it induces to a good or bad act: wherefore a habit is called good or bad by reason of the goodness or badness of its act: so that an act surpasses its habit in goodness or badness, since "the cause of a thing being such, is yet more so."
Reply to Objection 1. Nothing hinders one thing from standing above another simply, and below it in some respect. Now a thing is deemed above another simply if it surpasses it in a point which is proper to both; while it is deemed above it in a certain respect, if it surpasses it in something which is accidental to both. Now it has been shown from the very nature of act and habit, that act surpasses habit both in goodness and in badness. Whereas the fact that habit is more lasting than act, is accidental to them, and is due to the fact that they are both found in a nature such that it cannot always be in action, and whose action consists in a transient movement. Consequently act simply excels in goodness and badness, but habit excels in a certain respect.
Reply to Objection 2. A habit is several acts, not simply, but in a certain respect, i.e. virtually. Wherefore this does not prove that habit precedes act simply, both in goodness and in badness.
Reply to Objection 3. Habit causes act by way of efficient causality: but act causes habit, by way of final causality, in respect of which we consider the nature of good and evil. Consequently act surpasses habit both in goodness and in badness.
Source: Summa Theologica (New Advent)