I can't find an explicit statement that the seal of confession applies (or not) to the "confession" of a sin to be committed in the future, but all the evidence suggests that it does not.
Canon 983 and 984 are most relevant regarding the seal of confession:
Can. 983 §1. The sacramental seal is inviolable; therefore it is
absolutely forbidden for a confessor to betray in any way a penitent
in words or in any manner and for any reason.
§2. The interpreter, if there is one, and all others who in any way
have knowledge of sins from confession are also obliged to observe
Can. 984 §1. A confessor is prohibited completely from using knowledge
acquired from confession to the detriment of the penitent even when
any danger of revelation is excluded.
§2. A person who has been placed in authority cannot use in any manner
for external governance the knowledge about sins which he has received
in confession at any time.
Code of Canon Law
However, this presupposes that the confessor acquired this knowledge during a valid confession.
Regarding the essential parts of a valid confession, the Council of Trent taught that
the acts of the penitent himself, namely, contrition, confession and satisfaction, constitute the matter of this sacrament, which acts, inasmuch as they are by God's institution required in the penitent for the integrity of the sacrament and for the full and complete remission of sins, are for this reason called the parts of penance.
Contrition, which holds the first place among the aforesaid acts of the penitent, is a sorrow of mind and a detestation for sin committed with the purpose of not sinning in the future. This feeling of contrition was at all times necessary for obtaining the forgiveness of sins and thus indeed it prepares one who has fallen after baptism for the remission of sins, if it is united with confidence in the divine mercy and with the desire to perform the other things that are required to receive this sacrament in the proper manner. The holy council declares therefore, that this contrition implies not only an abstention from sin and the resolution and beginning of a new life, but also a hatred of the old...
Fourteenth Session of the Council of Trent
Contrition in the penitent is required for the integrity of the sacrament, and contrition is obviously lacking in the case at hand since the "penitent" is not actually sorrowful and intending to avoid the sin in the future. Since it is not a valid confession it follows that the seal of confession does not apply. After all, if a priest overhears the villain disclosing his criminal plans outside of the confessional (e.g. discussing plans with an accomplice) he is obviously not bound to secrecy just because he happens to be a priest.
Even if the seal of confession did apply the priest is able to take certain measures to prevent the crime without violating the seal. I found an interesting blog post about the seal of confession (supposedly written by a canon lawyer) which explores some hypothetical situations similar to the one posed, the most relevant of which is:
If the penitent is not willing to cooperate, there are sometimes
situations in which priests can find ways to help the authorities
without revealing the content of a person’s confession. If a penitent
has indicated, for example, that he fully intends to kill or harm
Person X, a priest may be able to warn the police that Person X is in
danger, but without fully explaining how he obtained this information.
I personally know of a case in which police received a phone call from
a priest, warning them that two teenaged sisters were in danger at
that very moment. The police understood that the priest was not
permitted to give them more specific information, and simply located
the girls, notified their parents, and made sure they were protected.
It is quite likely that some horrible crime was averted by this
priest’s action, yet he did not violate the sacramental seal—in fact,
nobody was really sure if he had learned the information in the
confessional or in a confidential conversation outside of it. Once
again, such collaboration between the authorities and the clergy
happens more often than we may realize.
Although the seal of confession probably does not apply in this situation, the penalty for violating the seal of confession is so severe that the priest may nonetheless choose to remain silent in order to make sure he does not violate the seal of confession.