Objection 1. It seems that the validity of a sacrament requires a good intention in the minister. For the minister's intention should be in conformity with the Church's intention, as explained above (8, ad 1). But the intention of the Church is always good. Therefore the validity of a sacrament requires of necessity a good intention in the minister.
Objection 2. Further, a perverse intention seems worse than a playful one. But a playful intention destroys a sacrament: for instance, if someone were to baptize anybody not seriously but in fun. Much more, therefore, does a perverse intention destroy a sacrament: for instance, if somebody were to baptize a man in order to kill him afterwards.
Objection 3. Further, a perverse intention vitiates the whole work, according to Luke 11:34: "If thy eye be evil, thy" whole "body will be darksome." But the sacraments of Christ cannot be contaminated by evil men; as Augustine says against Petilian (Cont. Litt. Petil ii). Therefore it seems that, if the minister's intention is perverse, the sacrament is invalid.
On the contrary, A perverse intention belongs to the wickedness of the minister. But the wickedness of the minister does not annul the sacrament: neither, therefore, does his perverse intention.
I answer that, The minister's intention may be perverted in two ways. First in regard to the sacrament: for instance, when a man does not intend to confer a sacrament, but to make a mockery of it. Such a perverse intention takes away the truth of the sacrament, especially if it be manifested outwardly.
Secondly, the minister's intention may be perverted as to something that follows the sacrament: for instance, a priest may intend to baptize a woman so as to be able to abuse her; or to consecrate the Body of Christ, so as to use it for sorcery. And because that which comes first does not depend on that which follows, consequently such a perverse intention does not annul the sacrament; but the minister himself sins grievously in having such an intention.
Reply to Objection 1. The Church has a good intention both as to the validity of the sacrament and as to the use thereof: but it is the former intention that perfects the sacrament, while the latter conduces to the meritorious effect. Consequently, the minister who conforms his intention to the Church as to the former rectitude, but not as to the latter, perfects the sacrament indeed, but gains no merit for himself.
Reply to Objection 2. The intention of mimicry or fun excludes the first kind of right intention, necessary for the validity of a sacrament. Consequently, there is no comparison.
Reply to Objection 3. A perverse intention perverts the action of the one who has such an intention, not the action of another. Consequently, the perverse intention of the minister perverts the sacrament in so far as it is his action: not in so far as it is the action of Christ, Whose minister he is. It is just as if the servant [minister] of some man were to carry alms to the poor with a wicked intention, whereas his master had commanded him with a good intention to do so.
Source: Summa Theologica (New Advent)