[Baljhin]

> I seem to remember a story of a mafia don who turned out to be an informant, which seems wild to me.

There's also "Jimmy the Weasel" Fratianno who turned FBI informant. In Ovid Demaris' book "The Last Mafioso: The Treacherous World of Jimmy Fratianno", he quotes Jimmy even laughs about being one, taking money from FBI and still running his rackets for a time.