Fallen from Grace
The Omertà - The Code of the Cosa Nostra
The Omertà implies “the categorical prohibition of cooperation with state authorities or reliance on its services, even when one has been victim of a crime”. A person should absolutely avoid interfering in the business of others and should not inform the authorities of a crime under any circumstances (though if justified he may personally avenge a physical attack on himself or on his family by vendetta, literally a taking of revenge, a feud). Even if somebody is convicted of a crime he has not committed, he is supposed to serve the sentence without giving the police any information about the real criminal, even if that criminal has nothing to do with the Mafia.
Within Mafia culture, breaking omertà is punishable by death.