So much so that if a penitent came and confessed that he was a serial killer who had murdered 100 people, and the confessor judged him likely to murder more, he could not report the man to the authorities.
I'm trying to jog my memory, but I recall recently seeing a story where some "confessor" ratted out a penitent and had him arrested for some crime.
If the confessor judged him likely to murder again, would that not make the confession null because the penitent lacks the firm purpose of sinning no more? Would the seal of confession still apply here?
Somewhat unrelated, but an interesting thing I read today that I hadn't thought of before:
"May a priest in a court of justice make known what he has learned in the confessional?
No; and if he is questioned, he ought to declare to the judge, with an oath if that be necessary, that he knows nothing of the subject in question. For what he learns through the confession of a penitent, he knows not as man, but as a representative of God, and in this quality he is not subject to the jurisdiction of the judge."
From the Exposition of Christian Doctrine Part III "Worship"