Objection 1. It would seem that scandal is not a special sin. For scandal is "something said or done less rightly." But this applies to every kind of sin. Therefore every sin is a scandal, and consequently, scandal is not a special sin.
Objection 2. Further, every special kind of sin, or every special kind of injustice, may be found separately from other kinds, as stated in Ethic. v, 3,5. But scandal is not to be found separately from other sins. Therefore it is not a special kind of sin.
Objection 3. Further, every special sin is constituted by something which specifies the moral act. But the notion of scandal consists in its being something done in the presence of others: and the fact of a sin being committed openly, though it is an aggravating circumstance, does not seem to constitute the species of a sin. Therefore scandal is not a special sin.
On the contrary, A special virtue has a special sin opposed to it. But scandal is opposed to a special virtue, viz. charity. For it is written (Romans 14:15): "If, because of thy meat, thy brother be grieved, thou walkest not now according to charity." Therefore scandal is a special sin.
I answer that, As stated above (Article 2), scandal is twofold, active and passive. Passive scandal cannot be a special sin, because through another's word or deed a man may fall into any kind of sin: and the fact that a man takes occasion to sin from another's word or deed, does not constitute a special kind of sin, because it does not imply a special deformity in opposition to a special virtue.
On the other hand, active scandal may be understood in two ways, directly and accidently. The scandal is accidental when it is beside the agent's intention, as when a man does not intend, by his inordinate deed or word, to occasion another's spiritual downfall, but merely to satisfy his own will. On such a case even active scandal is not a special sin, because a species is not constituted by that which is accidental.
Active scandal is direct when a man intends, by his inordinate word or deed, to draw another into sin, and then it becomes a special kind of sin on account of the intention of a special kind of end, because moral actions take their species from their end, as stated above (I-II, 1, 3; I-II, 18, 4,6). Hence, just as theft and murder are special kinds of sin, on account of their denoting the intention of doing a special injury to one's neighbor: so too, scandal is a special kind of sin, because thereby a man intends a special harm to his neighbor, and it is directly opposed to fraternal correction, whereby a man intends the removal of a special kind of harm.
Reply to Objection 1. Any sin may be the matter of active scandal, but it may derive the formal aspect of a special sin from the end intended, as stated above.
Reply to Objection 2. Active scandal can be found separate from other sins, as when a man scandalizes his neighbor by a deed which is not a sin in itself, but has an appearance of evil.
Reply to Objection 3. Scandal does not derive the species of a special sin from the circumstance in question, but from the intention of the end, as stated above.
Source: Summa Theologica (New Advent)