Objection 1. It seems that fear does not excuse from sin. For fear is a sin, as stated above (Article 1). But sin does not excuse from sin, rather does it aggravate it. Therefore fear does not excuse from sin.
Objection 2. Further, if any fear excuses from sin, most of all would this be true of the fear of death, to which, as the saying is, a courageous man is subject. Yet this fear, seemingly, is no excuse, because, since death comes, of necessity, to all, it does not seem to be an object of fear. Therefore fear does not excuse from sin.
Objection 3. Further, all fear is of evil, either temporal or spiritual. Now fear of spiritual evil cannot excuse sin, because instead of inducing one to sin, it withdraws one from sin: and fear of temporal evil does not excuse from sin, because according to the Philosopher (Ethic. iii, 6), "one should not fear poverty, nor sickness, nor anything that is not a result of one's own wickedness." Therefore it seems that in no sense does fear excuse from sin.
On the contrary, It is stated in the Decretals (I, 1, Cap. Constat.): "A man who has been forcibly and unwillingly ordained by heretics, has an ostensible excuse."
I answer that, As stated above (Article 3), fear is sinful in so far as it runs counter to the order of reason. Now reason judges certain evils to be shunned rather than others. Wherefore it is no sin not to shun what is less to be shunned in order to avoid what reason judges to be more avoided: thus death of the body is more to be avoided than the loss of temporal goods. Hence a man would be excused from sin if through fear of death he were to promise or give something to a robber, and yet he would be guilty of sin were he to give to sinners, rather than to the good to whom he should give in preference. On the other hand, if through fear a man were to avoid evils which according to reason are less to be avoided, and so incur evils which according to reason are more to be avoided, he could not be wholly excused from sin, because such like fear would be inordinate. Now the evils of the soul are more to be feared than the evils of the body. and evils of the body more than evils of external things. Wherefore if one were to incur evils of the soul, namely sins, in order to avoid evils of the body, such as blows or death, or evils of external things, such as loss of money; or if one were to endure evils of the body in order to avoid loss of money, one would not be wholly excused from sin. Yet one's sin would be extenuated somewhat, for what is done through fear is less voluntary, because when fear lays hold of a man he is under a certain necessity of doing a certain thing. Hence the Philosopher (Ethic. iii, 1) says that these things that are done through fear are not simply voluntary, but a mixture of voluntary and involuntary.
Reply to Objection 1. Fear excuses, not in the point of its sinfulness, but in the point of its involuntariness.
Reply to Objection 2. Although death comes, of necessity, to all, yet the shortening of temporal life is an evil and consequently an object of fear.
Reply to Objection 3. According to the opinion of Stoics, who held temporal goods not to be man's goods, it follows in consequence that temporal evils are not man's evils, and that therefore they are nowise to be feared. But according to Augustine (De Lib. Arb. ii) these temporal things are goods of the least account, and this was also the opinion of the Peripatetics. Hence their contraries are indeed to be feared; but not so much that one ought for their sake to renounce that which is good according to virtue.
Source: Summa Theologica (New Advent)