Objection 1. It would seem that no one sins purposely, or through certain malice. Because ignorance is opposed to purpose or certain malice. Now "every evil man is ignorant," according to the Philosopher (Ethic. iii, 1); and it is written (Proverbs 14:22): "They err that work evil." Therefore no one sins through certain malice.
Objection 2. Further, Dionysius says (Div. Nom. iv) that "no one works intending evil." Now to sin through malice seems to denote the intention of doing evil [Alluding to the derivation of "malitia" (malice) from "malum" (evil)] in sinning, because an act is not denominated from that which is unintentional and accidental. Therefore no one sins through malice.
Objection 3. Further, malice itself is a sin. If therefore malice is a cause of sin, it follows that sin goes on causing sin indefinitely, which is absurd. Therefore no one sins through malice.
On the contrary, It is written (Job 34:27): "[Who] as it were on purpose have revolted from God [Vulgate: 'Him'], and would not understand all His ways." Now to revolt from God is to sin. Therefore some sin purposely or through certain malice.
I answer that, Man like any other being has naturally an appetite for the good; and so if his appetite incline away to evil, this is due to corruption or disorder in some one of the principles of man: for it is thus that sin occurs in the actions of natural things. Now the principles of human acts are the intellect, and the appetite, both rational (i.e. the will) and sensitive. Therefore even as sin occurs in human acts, sometimes through a defect of the intellect, as when anyone sins through ignorance, and sometimes through a defect in the sensitive appetite, as when anyone sins through passion, so too does it occur through a defect consisting in a disorder of the will. Now the will is out of order when it loves more the lesser good. Again, the consequence of loving a thing less is that one chooses to suffer some hurt in its regard, in order to obtain a good that one loves more: as when a man, even knowingly, suffers the loss of a limb, that he may save his life which he loves more. Accordingly when an inordinate will loves some temporal good, e.g. riches or pleasure, more than the order of reason or Divine law, or Divine charity, or some such thing, it follows that it is willing to suffer the loss of some spiritual good, so that it may obtain possession of some temporal good. Now evil is merely the privation of some good; and so a man wishes knowingly a spiritual evil, which is evil simply, whereby he is deprived of a spiritual good, in order to possess a temporal good: wherefore he is said to sin through certain malice or on purpose, because he chooses evil knowingly.
Reply to Objection 1. Ignorance sometimes excludes the simple knowledge that a particular action is evil, and then man is said to sin through ignorance: sometimes it excludes the knowledge that a particular action is evil at this particular moment, as when he sins through passion: and sometimes it excludes the knowledge that a particular evil is not to be suffered for the sake of possessing a particular good, but not the simple knowledge that it is an evil: it is thus that a man is ignorant, when he sins through certain malice.
Reply to Objection 2. Evil cannot be intended by anyone for its own sake; but it can be intended for the sake of avoiding another evil, or obtaining another good, as stated above: and in this case anyone would choose to obtain a good intended for its own sake, without suffering loss of the other good; even as a lustful man would wish to enjoy a pleasure without offending God; but with the two set before him to choose from, he prefers sinning and thereby incurring God's anger, to being deprived of the pleasure.
Reply to Objection 3. The malice through which anyone sins, may be taken to denote habitual malice, in the sense in which the Philosopher (Ethic. v, 1) calls an evil habit by the name of malice, just as a good habit is called virtue: and in this way anyone is said to sin through malice when he sins through the inclination of a habit. It may also denote actual malice, whether by malice we mean the choice itself of evil (and thus anyone is said to sin through malice, in so far as he sins through making a choice of evil), or whether by malice we mean some previous fault that gives rise to a subsequent fault, as when anyone impugns the grace of his brother through envy. Nor does this imply that a thing is its own cause: for the interior act is the cause of the exterior act, and one sin is the cause of another; not indefinitely, however, since we can trace it back to some previous sin, which is not caused by any previous sin, as was explained above (75, 4, ad 3).
Source: Summa Theologica (New Advent)