I would like to add here that the priest is bound by his vows to uphold the sanctity of the confessional. There is a similarity in the oath taken by a doctor in which he cannot reveal anything that the patient does not allow him to reveal.
As for criminal law, no individual is obligated to report a crime to the police and you cannot be prosecuted for not reporting a crime. There’s no legal obligation on the priest, both as a man — and certainly not as a priest — because the priest could not reveal what he was told.
The difference between a lay person and a priest, in this scenario of a confession to a murder, is that we are only obligated to speak to the police if we are questioned. Only then, we cannot withhold information.
But, unless or until the police approach you about the crime (then you must answer their questions or be charged with obstruction), you are still under no obligation to report anything you might know or have been told. That’s our law. A priest has no such obligation to cooperate if approached or questioned about what might have been confessed to him. As a man he has no legal obligation to report knowledge of a crime. As a priest — he cannot.
And, yes, the priest would encourage and support the criminal to go to the police to turn himself in. However, I believe if a priest violated the sanctity of the confessional he could be stripped of his holy orders.
If our laws could violate the confessional, that also would violate our Constitution in which our government has no business in the beliefs of a religion. The very Constitution that allows us to practice our faiths freely, also protects that faith from government (in this case the police as representatives of government), interference in religious practices, such as any govenrment attempt to violate the sacrament of reconciliation (in this example).
Of course, if the criminal decided not to go to the police to turn himself in then it would be possible that he “got away with it.” However, that is between him and God — the police could not force the priest to reveal what he learned in confession and no judge would allow it. Separation of Church and state.