Objection 1. It would seem that it is not more grievous to sin through certain malice than through passion. Because ignorance excuses from sin either altogether or in part. Now ignorance is greater in one who sins through certain malice, than in one who sins through passion; since he that sins through certain malice suffers from the worst form of ignorance, which according to the Philosopher (Ethic. vii, 8) is ignorance of principle, for he has a false estimation of the end, which is the principle in matters of action. Therefore there is more excuse for one who sins through certain malice, than for one who sins through passion.
Objection 2. Further, the more a man is impelled to sin, the less grievous his sin, as is clear with regard to a man who is thrown headlong into sin by a more impetuous passion. Now he that sins through certain malice, is impelled by habit, the impulse of which is stronger than that of passion. Therefore to sin through habit is less grievous than to sin through passion.
Objection 3. Further, to sin through certain malice is to sin through choosing evil. Now he that sins through passion, also chooses evil. Therefore he does not sin less than the man who sins through certain malice.
On the contrary, A sin that is committed on purpose, for this very reason deserves heavier punishment, according to Job 34:26: "He hath struck them as being wicked, in open sight, who, as it were, on purpose, have revolted from Him." Now punishment is not increased except for a graver fault. Therefore a sin is aggravated through being done on purpose, i.e. through certain malice.
I answer that, A sin committed through malice is more grievous than a sin committed through passion, for three reasons. First, because, as sin consists chiefly in an act of the will, it follows that, other things being equal, a sin is all the more grievous, according as the movement of the sin belongs more to the will. Now when a sin is committed through malice, the movement of sin belongs more to the will, which is then moved to evil of its own accord, than when a sin is committed through passion, when the will is impelled to sin by something extrinsic, as it were. Wherefore a sin is aggravated by the very fact that it is committed through certain malice, and so much the more, as the malice is greater; whereas it is diminished by being committed through passion, and so much the more, as the passion is stronger. Secondly, because the passion which incites the will to sin, soon passes away, so that man repents of his sin, and soon returns to his good intentions; whereas the habit, through which a man sins, is a permanent quality, so that he who sins through malice, abides longer in his sin. For this reason the Philosopher (Ethic. vii, 8) compares the intemperate man, who sins through malice, to a sick man who suffers from a chronic disease, while he compares the incontinent man, who sins through passion, to one who suffers intermittently. Thirdly, because he who sins through certain malice is ill-disposed in respect of the end itself, which is the principle in matters of action; and so the defect is more dangerous than in the case of the man who sins through passion, whose purpose tends to a good end, although this purpose is interrupted on account of the passion, for the time being. Now the worst of all defects is defect of principle. Therefore it is evident that a sin committed through malice is more grievous than one committed through passion.
Reply to Objection 1. Ignorance of choice, to which the objection refers, neither excuses nor diminishes a sin, as stated above (Question 76, Article 4). Therefore neither does a greater ignorance of the kind make a sin to be less grave.
Reply to Objection 2. The impulse due to passion, is, as it were, due to a defect which is outside the will: whereas, by a habit, the will is inclined from within. Hence the comparison fails.
Reply to Objection 3. It is one thing to sin while choosing, and another to sin through choosing. For he that sins through passion, sins while choosing, but not through choosing, because his choosing is not for him the first principle of his sin; for he is induced through the passion, to choose what he would not choose, were it not for the passion. On the other hand, he that sins through certain malice, chooses evil of his own accord, in the way already explained (2,3), so that his choosing, of which he has full control, is the principle of his sin: and for this reason he is said to sin "through" choosing.
Source: Summa Theologica (New Advent)