Objection 1. It would seem that passion excuses from sin altogether. For whatever causes an act to be involuntary, excuses from sin altogether. But concupiscence of the flesh, which is a passion, makes an act to be involuntary, according to Galatians 5:17: "The flesh lusteth against the spirit... so that you do not the things that you would." Therefore passion excuses from sin altogether.
Objection 2. Further, passion causes a certain ignorance of a particular matter, as stated above (2; 76, 3). But ignorance of a particular matter excuses from sin altogether, as stated above (Question 6, Article 8). Therefore passion excuses from sin altogether.
Objection 3. Further, disease of the soul is graver than disease of the body. But bodily disease excuses from sin altogether, as in the case of mad people. Much more, therefore, does passion, which is a disease of the soul.
On the contrary, The Apostle (Romans 7:5) speaks of the passions as "passions of sins," for no other reason than that they cause sin: which would not be the case if they excused from sin altogether. Therefore passion does not excuse from sin altogether.
I answer that, An act which, in its genus, is evil, cannot be excused from sin altogether, unless it be rendered altogether involuntary. Consequently, if the passion be such that it renders the subsequent act wholly involuntary, it entirely excuses from sin; otherwise, it does not excuse entirely. In this matter two points apparently should be observed: first, that a thing may be voluntary either "in itself," as when the will tends towards it directly; or "in its cause," when the will tends towards that cause and not towards the effect; as is the case with one who wilfully gets drunk, for in that case he is considered to do voluntarily whatever he does through being drunk. Secondly, we must observe that a thing is said to be voluntary "directly" or "indirectly"; directly, if the will tends towards it; indirectly, if the will could have prevented it, but did not.
Accordingly therefore we must make a distinction: because a passion is sometimes so strong as to take away the use of reason altogether, as in the case of those who are mad through love or anger; and then if such a passion were voluntary from the beginning, the act is reckoned a sin, because it is voluntary in its cause, as we have stated with regard to drunkenness. If, however, the cause be not voluntary but natural, for instance, if anyone through sickness or some such cause fall into such a passion as deprives him of the use of reason, his act is rendered wholly involuntary, and he is entirely excused from sin. Sometimes, however, the passion is not such as to take away the use of reason altogether; and then reason can drive the passion away, by turning to other thoughts, or it can prevent it from having its full effect; since the members are not put to work, except by the consent of reason, as stated above (Question 17, Article 9): wherefore such a passion does not excuse from sin altogether.
Reply to Objection 1. The words, "So that you do not the things that you would" are not to be referred to outward deeds, but to the inner movement of concupiscence; for a man would wish never to desire evil, in which sense we are to understand the words of Romans 7:19: "The evil which I will not, that I do." Or again they may be referred to the will as preceding the passion, as is the case with the incontinent, who act counter to their resolution on account of their concupiscence.
Reply to Objection 2. The particular ignorance which excuses altogether, is ignorance of a circumstance, which a man is unable to know even after taking due precautions. But passion causes ignorance of law in a particular case, by preventing universal knowledge from being applied to a particular act, which passion the reason is able to drive away, as stated.
Reply to Objection 3. Bodily disease is involuntary: there would be a comparison, however, if it were voluntary, as we have stated about drunkenness, which is a kind of bodily disease.
Source: Summa Theologica (New Advent)