Objection 1. It would seem that whoever sins through certain malice, sins through habit. For the Philosopher says (Ethic. v, 9) that "an unjust action is not done as an unjust man does it," i.e. through choice, "unless it be done through habit." Now to sin through certain malice is to sin through making a choice of evil, as stated above (Article 1). Therefore no one sins through certain malice, unless he has the habit of sin.
Objection 2. Further, Origen says (Peri Archon iii) that "a man is not suddenly ruined and lost, but must needs fall away little by little." But the greatest fall seems to be that of the man who sins through certain malice. Therefore a man comes to sin through certain malice, not from the outset, but from inveterate custom, which may engender a habit.
Objection 3. Further, whenever a man sins through certain malice, his will must needs be inclined of itself to the evil he chooses. But by the nature of that power man is inclined, not to evil but to good. Therefore if he chooses evil, this must be due to something supervening, which is passion or habit. Now when a man sins through passion, he sins not through certain malice, but through weakness, as stated (77, 3). Therefore whenever anyone sins through certain malice, he sins through habit.
On the contrary, The good habit stands in the same relation to the choice of something good, as the bad habit to the choice of something evil. But it happens sometimes that a man, without having the habit of a virtue, chooses that which is good according to that virtue. Therefore sometimes also a man, without having the habit of a vice, may choose evil, which is to sin through certain malice.
I answer that, The will is related differently to good and to evil. Because from the very nature of the power, it is inclined to the rational good, as its proper object; wherefore every sin is said to be contrary to nature. Hence, if a will be inclined, by its choice, to some evil, this must be occasioned by something else. Sometimes, in fact, this is occasioned through some defect in the reason, as when anyone sins through ignorance; and sometimes this arises through the impulse of the sensitive appetite, as when anyone sins through passion. Yet neither of these amounts to a sin through certain malice; for then alone does anyone sin through certain malice, when his will is moved to evil of its own accord. This may happen in two ways. First, through his having a corrupt disposition inclining him to evil, so that, in respect of that disposition, some evil is, as it were, suitable and similar to him; and to this thing, by reason of its suitableness, the will tends, as to something good, because everything tends, of its own accord, to that which is suitable to it. Moreover this corrupt disposition is either a habit acquired by custom, or a sickly condition on the part of the body, as in the case of a man who is naturally inclined to certain sins, by reason of some natural corruption in himself. Secondly, the will, of its own accord, may tend to an evil, through the removal of some obstacle: for instance, if a man be prevented from sinning, not through sin being in itself displeasing to him, but through hope of eternal life, or fear of hell, if hope give place to despair, or fear to presumption, he will end in sinning through certain malice, being freed from the bridle, as it were.
It is evident, therefore, that sin committed through certain malice, always presupposes some inordinateness in man, which, however, is not always a habit: so that it does not follow of necessity, if a man sins through certain malice, that he sins through habit.
Reply to Objection 1. To do an action as an unjust man does, may be not only to do unjust things through certain malice, but also to do them with pleasure, and without any notable resistance on the part of reason, and this occurs only in one who has a habit.
Reply to Objection 2. It is true that a man does not fall suddenly into sin from certain malice, and that something is presupposed; but this something is not always a habit, as stated above.
Reply to Objection 3. That which inclines the will to evil, is not always a habit or a passion, but at times is something else. Moreover, there is no comparison between choosing good and choosing evil: because evil is never without some good of nature, whereas good can be perfect without the evil of fault.
Source: Summa Theologica (New Advent)