Objection 1. It would seem that sin committed through passion cannot be mortal. Because venial sin is condivided with mortal sin. Now sin committed from weakness is venial, since it has in itself a motive for pardon [venia]. Since therefore sin committed through passion is a sin of weakness, it seems that it cannot be mortal.
Objection 2. Further, the cause is more powerful than its effect. But passion cannot be a mortal sin, for there is no mortal sin in the sensuality, as stated above (Question 74, Article 4). Therefore a sin committed through passion cannot be mortal.
Objection 3. Further, passion is a hindrance to reason, as explained above (1,2). Now it belongs to the reason to turn to God, or to turn away from Him, which is the essence of a mortal sin. Therefore a sin committed through passion cannot be mortal.
On the contrary, The Apostle says (Romans 7:5) that "the passions of the sins... work [Vulgate: 'did work'] in our members to bring forth fruit unto death." Now it is proper to mortal sin to bring forth fruit unto death. Therefore sin committed through passion may be mortal.
I answer that, Mortal sin, as stated above (Question 72, Article 5), consists in turning away from our last end which is God, which aversion pertains to the deliberating reason, whose function it is also to direct towards the end. Therefore that which is contrary to the last end can happen not to be a mortal sin, only when the deliberating reason is unable to come to the rescue, which is the case in sudden movements. Now when anyone proceeds from passion to a sinful act, or to a deliberate consent, this does not happen suddenly: and so the deliberating reason can come to the rescue here, since it can drive the passion away, or at least prevent it from having its effect, as stated above: wherefore if it does not come to the rescue, there is a mortal sin; and it is thus, as we see, that many murders and adulteries are committed through passion.
Reply to Objection 1. A sin may be venial in three ways. First, through its cause, i.e. through having cause to be forgiven, which cause lessens the sin; thus a sin that is committed through weakness or ignorance is said to be venial. Secondly, through its issue; thus every sin, through repentance, becomes venial, i.e. receives pardon [veniam]. Thirdly, by its genus, e.g. an idle word. This is the only kind of venial sin that is opposed to mortal sin: whereas the objection regards the first kind.
Reply to Objection 2. Passion causes sin as regards the adherence to something. But that this be a mortal sin regards the aversion, which follows accidentally from the adherence, as stated above (6, ad 1): hence the argument does not prove.
Reply to Objection 3. Passion does not always hinder the act of reason altogether: consequently the reason remains in possession of its free-will, so as to turn away from God, or turn to Him. If, however, the use of reason be taken away altogether, the sin is no longer either mortal or venial.
Source: Summa Theologica (New Advent)