Does the seal of confession apply even when the sin is intended and there is no desire to repent?

Does the seal of confession apply even when the sin is intended and there is no desire to repent?

The short answer is yes.

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 secrecy.

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

The seal of confession is inviolable, regardless if the penitent receives absolution or not. Canon Law does not give or state an exception for unforgiven sin. Thus a confessor must guard the secret of confessed sins whether sacramentally absolved or not.

Unless the Code of Canon Law states otherwise (which it does not) a priest can not betray the penitent in any way which could harm him is some way, even if the penitent has been denied sacramental absolution.

Sins confessed make the seal of confession an obligation. Regardless if the penitent receives absolution or not is immaterial, the seal of confession can not be broken. Contrition is essentially necessary for absolution, the priest is still obliged to keep the seal of confession. The seal of confession and the sacramentally forgiven sin are two distinct part of confession.

The sacramental seal is inviolable. Quoting Canon 983.1 of the Code of Canon Law, the Catechism states, “…It is a crime for a confessor in any way to betray a penitent by word or in any other manner or for any reason” (#2490). A priest, therefore, cannot break the seal to save his own life, to protect his good name, to refute a false accusation, to save the life of another, to aid the course of justice (like reporting a crime), or to avert a public calamity. He cannot be compelled by law to disclose a person’s confession or be bound by any oath he takes, e.g. as a witness in a court trial. A priest cannot reveal the contents of a confession either directly, by repeating the substance of what has been said, or indirectly, by some sign, suggestion, or action. A Decree from the Holy Office (November 18, 1682) mandated that confessors are forbidden, even where there would be no revelation direct or indirect, to make any use of the knowledge obtained in the confession that would “displease” the penitent or reveal his identity.

There are certain circumstances in which the priest would have to discuss the matter of a confession with another but would do so without revealing the identity of the person. For instance, some sins are so grievous that the priest must ask for permission from a superior to grant absolution. For example, if a person desecrates the Holy Eucharist in some sacrilegious act or just “throws away” the Holy Eucharist, he incurs an automatic excommunication reserved to the Apostolic See (Canon 1367); therefore, when this sin is confessed, a priest must inform the penitent that he must contact the Bishop, who would obtain from the Apostolic See the proper permission for the absolution of the sin and the lifting of the ban of excommunication. While keeping the seal of confession, some arrangement would have to be made for the penitent to return to the priest and receive absolution and the appropriate penance.

Or, if a priest needs guidance from a more experienced confessor to deal with a difficult case of conscience brought to him in confession, he first must ask the permission of the penitent to discuss the matter and make arrangements for another meeting. Here again, the priest must keep the identity of the person secret.

Sometimes a penitent wants to discuss the subject matter of a previous confession– a particular sin, fault, temptation, circumstance– in a counseling session or in a conversation with the same priest. Respecting the seal of confession, the priest would have to ask the penitent to refresh his memory, so as to revisit the particulars again outside of confession. For example, especially with the advent of “face-to-face confession,” I have had individuals come up to me and say, “Father, remember that problem I spoke to you about in confession?” I have to say, “Please refresh my memory.” - Can the seal of confession be broken or the secrets ever be revealed by priests?

The seal of confession applies to any sin confessed, as the canons in Null's answer show.

Intention to sin is itself a sin. Even taking delight in sin, without the intention of committing the sin, is a sin, called morose delectation:

Deliberate complacency in a sinful object, presented by the imagination but unaccompanied by a desire for the object. Also called morose delectation. (morosa delectatio).

Sorrow for sins, even if it is only "animated by a supernatural motive that is less than a perfect love of God," is necessary for a valid confession.

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 secrecy.

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.