A priest may (and in some cases is obliged to) withhold absolution until the person confesses his or her crime to the civil authorities. They can be pretty forceful with that tack. (Read Giovanni Guareschi's Little World of Don Camillo, where a Communist assassin tries to get absolution from Don Camillo at gunpoint. He does not succeed.)
But the Seal is absolute. (By the way, a legal privilege is accorded in law to Protestant ministers, psychiatrists, psychologists, lawyers, accountants, and physicians. I guess they're all members of a cult?)
My point was that there has been, in the past, court challenges to this “Absolute seal”. If a priest hears of a mass murder about to happen, or has already happened, I would think he would make “the right” decision. If he is somehow confused about what is right and wrong, I think its time to get a new vocation.