Objection 1. It would seem that sin cannot be the punishment of sin. For the purpose of punishment is to bring man back to the good of virtue, as the Philosopher declares (Ethic. x, 9). Now sin does not bring man back to the good of virtue, but leads him in the opposite direction. Therefore sin is not the punishment of sin.
Objection 2. Further, just punishments are from God, as Augustine says (Qq. lxxxiii, qu. 82). But sin is not from God, and is an injustice. Therefore sin cannot be the punishment of sin.
Objection 3. Further, the nature of punishment is to be something against the will. But sin is something from the will, as shown above (74, A1,2). Therefore sin cannot be the punishment of sin.
On the contrary, Gregory speaks (Hom. xi in Ezech.) that some sins are punishments of others.
I answer that, We may speak of sin in two ways: first, in its essence, as such; secondly, as to that which is accidental thereto. Sin as such can nowise be the punishment of another. Because sin considered in its essence is something proceeding from the will, for it is from this that it derives the character of guilt. Whereas punishment is essentially something against the will, as stated in the I, 48, 5. Consequently it is evident that sin regarded in its essence can nowise be the punishment of sin.
On the other hand, sin can be the punishment of sin accidentally in three ways. First, when one sin is the cause of another, by removing an impediment thereto. For passions, temptations of the devil, and the like are causes of sin, but are impeded by the help of Divine grace which is withdrawn on account of sin. Wherefore since the withdrawal of grace is a punishment, and is from God, as stated above (Question 79, Article 3), the result is that the sin which ensues from this is also a punishment accidentally. It is in this sense that the Apostle speaks (Romans 1:24) when he says: "Wherefore God gave them up to the desires of their heart," i.e. to their passions; because, to wit, when men are deprived of the help of Divine grace, they are overcome by their passions. In this way sin is always said to be the punishment of a preceding sin. Secondly, by reason of the substance of the act, which is such as to cause pain, whether it be an interior act, as is clearly the case with anger or envy, or an exterior act, as is the case with one who endures considerable trouble and loss in order to achieve a sinful act, according to Wisdom 5:7: "We wearied ourselves in the way of iniquity." Thirdly, on the part of the effect, so that one sin is said to be a punishment by reason of its effect. In the last two ways, a sin is a punishment not only in respect of a preceding sin, but also with regard to itself.
Reply to Objection 1. Even when God punishes men by permitting them to fall into sin, this is directed to the good of virtue. Sometimes indeed it is for the good of those who are punished, when, to wit, men arise from sin, more humble and more cautious. But it is always for the amendment of others, who seeing some men fall from sin to sin, are the more fearful of sinning. With regard to the other two ways, it is evident that the punishment is intended for the sinner's amendment, since the very fact that man endures toil and loss in sinning, is of a nature to withdraw man from sin.
Reply to Objection 2. This objection considers sin essentially as such: and the same answer applies to the Third Objection.
Source: Summa Theologica (New Advent)