If somebody confesses to a priest that he has killed a number of people and he intends to kill some more people what is the priest to do? Could he even gives names or clues about what people this person said he would kill? Can the priest contact the intended victims or the police to try to stop the killings?
A priest cannot violate the seal of confession for any reason whatsoever. He can deny absolution to someone he believes is not truly repentant for his sins — and a stated intention to recommit the very sin being confessed during the act of sacramental confession itself could indicate impenitence — but he cannot in any way, either by word or action, violate the seal of confession. That means that not only can he not say anything, but he cannot act upon the information gained in the confession either. In the hypothetical you propose, such a priest could not contact authorities or victims, give clues, or — to give an example of a wordless action — steal the murderer’s weapon to render him weaponless.
The priest acts in persona Christi (“in the person of Christ”) and in confession the penitent is speaking to God himself through the ministry of the priest. That means that the information given during sacramental confession doesn’t properly belong to the priest himself as a fellow human being; it belongs properly to the penitent and to God. That is one reason why the priest cannot act upon what he hears in sacramental confession. Another reason is even more serious: If penitents have reason to fear that a priest is allowed to reveal their confessions, they won’t confess. If they don’t confess, they risk hell. Ultimately, the inviolability of the seal of confession is about saving lives — it is about saving the immortal souls of those who have committed mortal sin and are at risk of eternal damnation.
Michelle Arnold – Catholic Answers Apologist