Objection 1. It would seem that a circumstance does not aggravate a sin. Because sin takes its gravity from its species. Now a circumstance does not specify a sin, for it is an accident thereof. Therefore the gravity of a sin is not taken from a circumstance.
Objection 2. Further, a circumstance is either evil or not: if it is evil, it causes, of itself, a species of evil; and if it is not evil, it cannot make a thing worse. Therefore a circumstance nowise aggravates a sin.
Objection 3. Further, the malice of a sin is derived from its turning away (from God). But circumstances affect sin on the part of the object to which it turns. Therefore they do not add to the sin's malice.
On the contrary, Ignorance of a circumstance diminishes sin: for he who sins through ignorance of a circumstance, deserves to be forgiven (Ethic. iii, 1). Now this would not be the case unless a circumstance aggravated a sin. Therefore a circumstance makes a sin more grievous.
I answer that, As the Philosopher says in speaking of habits of virtue (Ethic. ii, 1,2), "it is natural for a thing to be increased by that which causes it." Now it is evident that a sin is caused by a defect in some circumstance: because the fact that a man departs from the order of reason is due to his not observing the due circumstances in his action. Wherefore it is evident that it is natural for a sin to be aggravated by reason of its circumstances. This happens in three ways. First, in so far as a circumstance draws a sin from one kind to another: thus fornication is the intercourse of a man with one who is not his wife: but if to this be added the circumstance that the latter is the wife of another, the sin is drawn to another kind of sin, viz. injustice, in so far as he usurps another's property; and in this respect adultery is a more grievous sin than fornication. Secondly, a circumstance aggravates a sin, not by drawing it into another genus, but only by multiplying the ratio of sin: thus if a wasteful man gives both when he ought not, and to whom he ought not to give, he commits the same kind of sin in more ways than if he were to merely to give to whom he ought not, and for that very reason his sin is more grievous; even as that sickness is the graver which affects more parts of the body. Hence Cicero says (Paradox. iii) that "in taking his father's life a man commits many sins; for he outrages one who begot him, who fed him, who educated him, to whom he owes his lands, his house, his position in the republic." Thirdly, a circumstance aggravates a sin by adding to the deformity which the sin derives from another circumstance: thus, taking another's property constitutes the sin of theft; but if to this be added the circumstance that much is taken of another's property, the sin will be more grievous; although in itself, to take more or less has not the character of a good or of an evil act.
Reply to Objection 1. Some circumstances do specify a moral act, as stated above (Question 18, Article 10). Nevertheless a circumstance which does not give the species, may aggravate a sin; because, even as the goodness of a thing is weighed, not only in reference to its species, but also in reference to an accident, so the malice of an act is measured, not only according to the species of that act, but also according to a circumstance.
Reply to Objection 2. A circumstance may aggravate a sin either way. For if it is evil, it does not follow that it constitutes the sin's species; because it may multiply the ratio of evil within the same species, as stated above. And if it be not evil, it may aggravate a sin in relation to the malice of another circumstance.
Reply to Objection 3. Reason should direct the action not only as regards the object, but also as regards every circumstance. Therefore one may turn aside from the rule of reason through corruption of any single circumstance; for instance, by doing something when one ought not or where one ought not; and to depart thus from the rule of reason suffices to make the act evil. This turning aside from the rule of reason results from man's turning away from God, to Whom man ought to be united by right reason.
Source: Summa Theologica (New Advent)