Salvatore Gravano (born 1945), known as Sammy the Bull, was the underboss of the Gambino crime family and John Gotti's right-hand man. He admitted to participating in 19 murders. His decision to cooperate with the government against Gotti was the most significant break of the Mafia's code of silence in history, resulting in Gotti's conviction. He was later re-arrested for running a drug operation in Arizona.

Reference:

Sammy Gravano on Wikipedia

https://en.wikipedia.org/wiki/Sammy_Gravano